Problem:
Score : 300 points

Problem Statement

There is a sequence of length N: A_1, A_2, ..., A_N. Initially, this sequence is a permutation of 1, 2, ..., N.

On this sequence, Snuke can perform the following operation:

Choose K consecutive elements in the sequence. Then, replace the value of each chosen element with the minimum value among the chosen elements.

Snuke would like to make all the elements in this sequence equal by repeating the operation above some number of times.
Find the minimum number of operations required.
It can be proved that, Under the constraints of this problem, this objective is always achievable.

Constraints

2 \leq K \leq N \leq 100000

A_1, A_2, ..., A_N is a permutation of 1, 2, ..., N.

Input

Input is given from Standard Input in the following format:

N K
A_1 A_2 ... A_N

Output

Print the minimum number of operations required.

Sample Input 1

4 3
2 3 1 4

Sample Output 1

2

One optimal strategy is as follows:

In the first operation, choose the first, second and third elements. The sequence A becomes 1, 1, 1, 4.

In the second operation, choose the second, third and fourth elements. The sequence A becomes 1, 1, 1, 1.

Sample Input 2

3 3
1 2 3

Sample Output 2

1

Sample Input 3

8 3
7 3 1 8 4 6 2 5

Sample Output 3

4

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.*;
import java.io.*;
import java.math.*;
public class Main {
	static boolean DEBUG;
	public static void main(String[] args) {
		DEBUG = args.length > 0 && args[0].equals("-DEBUG");
		Solver solver = new Solver();
		solver.solve();
		solver.exit();
	}

	static class FastScanner {
		private final InputStream in = System.in;
		private final byte[] buffer = new byte[1024];
		private int ptr = 0;
		private int buflen = 0;
		private boolean hasNextByte() {
			if(ptr < buflen) {
				return true;
			}else{
				ptr = 0;
				try {
					buflen = in.read(buffer);
				} catch (IOException e) {
					e.printStackTrace();
				}
				if(buflen <= 0) {
					return false;
				}
			}
			return true;
		}
		private int readByte() { if(hasNextByte()) return buffer[ptr++]; else return -1;}
		private boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}
		private void skipUnprintable() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;}
		public boolean hasNext() { skipUnprintable(); return hasNextByte();}
		public String next() {
			if(!hasNext()) throw new NoSuchElementException();
			StringBuilder sb = new StringBuilder();
			int b = readByte();
			while(isPrintableChar(b)) {
				sb.appendCodePoint(b);
				b = readByte();
			}
			return sb.toString();
		}
		public long nextLong() {
			if(!hasNext()) throw new NoSuchElementException();
			long n = 0;
			boolean minus = false;
			int b = readByte();
			if(b == '-') {
				minus = true;
				b = readByte();
			}
			if(b < '0' || '9' < b) {
				throw new NumberFormatException();
			}
			while(true){
				if('0' <= b && b <= '9') {
					n *= 10;
					n += b - '0';
				}else if(b == -1 || !isPrintableChar(b)){
					return minus ? -n : n;
				}else{
					throw new NumberFormatException();
				}
				b = readByte();
			}
		}
	}

	static class Solver {
		FastScanner sc = new FastScanner();
		public Solver() { }

		String ns() { return sc.next(); }
		String[] ns(int n) {
			String a[] = new String[n];
			for(int i = 0; i < n; i ++) { a[i] = ns(); }
			return a;
		}
		String[][] ns(int n, int m) {
			String a[][] = new String[n][m];
			for(int i = 0; i < n; i ++) { a[i] = ns(m); }
			return a;
		}
		char[] nc(int n) {
			String str = ns();
			if(n < 0) n = str.length();
			char a[] = new char[n];
			for(int i = 0; i < n; i ++) { a[i] = str.charAt(i); }
			return a;
		}
		char[][] nc(int n, int m) {
			char a[][] = new char[n][m];
			for(int i = 0; i < n; i ++) { a[i] = nc(m); }
			return a;
		}
		boolean[] nb(int n, char t) {
			char c[] = nc(-1);
			if(n < 0) n = c.length;
			boolean a[] = new boolean[n];
			for(int i = 0; i < n; i ++) { a[i] = c[i] == t; }
			return a;
		}
		boolean[][] nb(int n, int m, char t) {
			boolean a[][] = new boolean[n][m];
			for(int i = 0; i < n; i ++) { a[i] = nb(m, t); }
			return a;
		}
		int ni() { return (int)sc.nextLong(); }
		int[] ni(int n) {
			int a[] = new int[n];
			for(int i = 0; i < n; i ++) { a[i] = ni(); }
			return a;
		}
		int[][] ni(int n, int m) {
			int a[][] = new int[n][m];
			for(int i = 0; i < n; i ++) { a[i] = ni(m); }
			return a;
		}
		long nl() { return sc.nextLong(); }
		long[] nl(int n) {
			long a[] = new long[n];
			for(int i = 0; i < n; i ++) { a[i] = nl(); }
			return a;
		}
		long[][] nl(int n, int m) {
			long a[][] = new long[n][m];
			for(int i = 0; i < n; i ++) { a[i] = nl(m); }
			return a;
		}
		double nd() { return Double.parseDouble(sc.next()); }
		double[] nd(int n) {
			double a[] = new double[n];
			for(int i = 0; i < n; i ++) { a[i] = nd(); }
			return a;
		}
		double[][] nd(int n, int m) {
			double a[][] = new double[n][m];
			for(int i = 0; i < n; i ++) { a[i] = nd(m); }
			return a;
		}

		PrintWriter out = new PrintWriter(System.out);
		PrintWriter err = new PrintWriter(System.err);
		void prt() { out.print(""); }
		<T> void prt(T a) { out.print(a); }
		void prtln() { out.println(""); }
		<T> void prtln(T a) { out.println(a); }
		void prtln(int... a) {
			StringBuilder sb = new StringBuilder();
			for(int element : a){ sb.append(element+" "); }
			prtln(sb.toString().trim());
		}
		void prtln(long... a) {
			StringBuilder sb = new StringBuilder();
			for(long element : a){ sb.append(element+" "); }
			prtln(sb.toString().trim());
		}
		void prtln(double... a) {
			StringBuilder sb = new StringBuilder();
			for(double element : a){ sb.append(element+" "); }
			prtln(sb.toString().trim());
		}
		void prtln(String... a) {
			StringBuilder sb = new StringBuilder();
			for(String element : a){ sb.append(element+" "); }
			prtln(sb.toString().trim());
		}
		void prtln(char... a) {
			StringBuilder sb = new StringBuilder();
			for(char element : a){ sb.append(element); }
			prtln(sb.toString().trim());
		}
		void prtln(int[][] a) { for(int[] element : a){ prtln(element); } }
		void prtln(long[][] a) { for(long[] element : a){ prtln(element); } }
		void prtln(double[][] a) { for(double[] element : a){ prtln(element); } }
		void prtln(String[][] a) { for(String[] element : a){ prtln(element); } }
		void prtln(char[][] a) { for(char[] element : a){ prtln(element); } }
		String errconvert(int a) { return isINF(a) ? "_" : String.valueOf(a); }
		String errconvert(long a) { return isINF(a) ? "_" : String.valueOf(a); }
		void errprt(int a) { if(DEBUG) { err.print(errconvert(a)); } }
		void errprt(long a) { if(DEBUG) { err.print(errconvert(a)); } }
		void errprt() { if(DEBUG) { err.print(""); } }
		<T> void errprt(T a) { if(DEBUG) { err.print(a); } }
		void errprt(boolean a) { if(DEBUG) { errprt(a ? "#" : "."); } }
		void errprtln() { if(DEBUG) { err.println(""); } }
		void errprtln(int a) { if(DEBUG) { err.println(errconvert(a)); } }
		void errprtln(long a) { if(DEBUG) { err.println(errconvert(a)); } }
		<T> void errprtln(T a) { if(DEBUG) { err.println(a); } }
		void errprtln(boolean a) { if(DEBUG) { errprtln(a ? "#" : "."); } }
		void errprtln(int... a) {
			if(DEBUG) {
				StringBuilder sb = new StringBuilder();
				for(int element : a){ sb.append(errconvert(element)+" "); }
				errprtln(sb.toString().trim());
			}
		}
		void errprtln(long... a) {
			if(DEBUG) {
				StringBuilder sb = new StringBuilder();
				for(long element : a){ sb.append(errconvert(element)+" "); }
				errprtln(sb.toString().trim());
			}
		}
		void errprtln(double... a) {
			if(DEBUG) {
				StringBuilder sb = new StringBuilder();
				for(double element : a){ sb.append(element+" "); }
				errprtln(sb.toString().trim());
			}
		}
		void errprtln(String... a) {
			if(DEBUG) {
				StringBuilder sb = new StringBuilder();
				for(String element : a){ sb.append(element+" "); }
				errprtln(sb.toString().trim());
			}
		}
		void errprtln(char... a) {
			if(DEBUG) {
				StringBuilder sb = new StringBuilder();
				for(char element : a){ sb.append(element); }
				errprtln(sb.toString().trim());
			}
		}
		void errprtln(boolean... a) {
			if(DEBUG) {
				StringBuilder sb = new StringBuilder();
				for(boolean element : a){ sb.append((element ? "#" : ".")+" "); }
				errprtln(sb.toString().trim());
			}
		}
		void errprtln(int[][] a) { if(DEBUG) { for(int[] element : a){ errprtln(element); } } }
		void errprtln(long[][] a) { if(DEBUG) { for(long[] element : a){ errprtln(element); } } }
		void errprtln(double[][] a) { if(DEBUG) { for(double[] element : a){ errprtln(element); } } }
		void errprtln(String[][] a) { if(DEBUG) { for(String[] element : a){ errprtln(element); } } }
		void errprtln(char[][] a) { if(DEBUG) { for(char[] element : a){ errprtln(element); } } }
		void errprtln(boolean[][] a) { if(DEBUG) { for(boolean[] element : a){ errprtln(element); } } }

		void reply(boolean b) { prtln(b ? "Yes" : "No"); }
		void REPLY(boolean b) { prtln(b ? "YES" : "NO"); }

		void flush() { out.flush(); if(DEBUG) { err.flush(); } }
		void exit() { flush(); System.exit(0); }
		void assertion(boolean b) { if(!b) throw new AssertionError(); }

		int min(int a, int b) { return Math.min(a, b); }
		long min(long a, long b) { return Math.min(a, b); }
		double min(double a, double b) { return Math.min(a, b); }
		int min(int... x) {
			int min = x[0];
			for(int val : x) { min = min(min, val); }
			return min;
		}
		long min(long... x) {
			long min = x[0];
			for(long val : x) { min = min(min, val); }
			return min;
		}
		double min(double... x) {
			double min = x[0];
			for(double val : x) { min = min(min, val); }
			return min;
		}
		int max(int a, int b) { return Math.max(a, b); }
		long max(long a, long b) { return Math.max(a, b); }
		double max(double a, double b) { return Math.max(a, b); }
		int max(int... x) {
			int max = x[0];
			for(int val : x) { max = max(max, val); }
			return max;
		}
		long max(long... x) {
			long max = x[0];
			for(long val : x) { max = max(max, val); }
			return max;
		}
		double max(double... x) {
			double max = x[0];
			for(double val : x) { max = max(max, val); }
			return max;
		}
		long sum(int... a) {
			long sum = 0;
			for(int element : a) { sum += element; }
			return sum;
		}
		long sum(long... a) {
			long sum = 0;
			for(long element : a) { sum += element; }
			return sum;
		}
		double sum(double... a) {
			double sum = 0;
			for(double element : a) { sum += element; }
			return sum;
		}
		long sum(boolean... a) {
			long sum = 0;
			for(boolean element : a) { sum += element ? 1 : 0; }
			return sum;
		}
		long[] sums(int[] a) {
			long sum[] = new long[a.length + 1];
			sum[0] = 0;
			for(int i = 0; i < a.length; i ++) { sum[i + 1] = sum[i] + a[i]; }
			return sum;
		}
		long[] sums(long[] a) {
			long sum[] = new long[a.length + 1];
			sum[0] = 0;
			for(int i = 0; i < a.length; i ++) { sum[i + 1] = sum[i] + a[i]; }
			return sum;
		}
		double[] sums(double[] a) {
			double sum[] = new double[a.length + 1];
			sum[0] = 0;
			for(int i = 0; i < a.length; i ++) { sum[i + 1] = sum[i] + a[i]; }
			return sum;
		}
		long[] sums(boolean[] a) {
			long sum[] = new long[a.length + 1];
			sum[0] = 0;
			for(int i = 0; i < a.length; i ++) { sum[i + 1] = sum[i] + (a[i] ? 1 : 0); }
			return sum;
		}
		long[][] sums(int[][] a) {
			long sum[][] = new long[a.length + 1][a[0].length + 1];
			fill(sum, 0);
			for(int i = 0; i < a.length; i ++) {
				for(int j = 0; j < a[i].length; j ++) {
					sum[i + 1][j + 1] = sum[i + 1][j] + sum[i][j + 1] - sum[i][j] + a[i][j];
				}
			}
			return sum;
		}
		long[][] sums(long[][] a) {
			long sum[][] = new long[a.length + 1][a[0].length + 1];
			fill(sum, 0);
			for(int i = 0; i < a.length; i ++) {
				for(int j = 0; j < a[i].length; j ++) {
					sum[i + 1][j + 1] = sum[i + 1][j] + sum[i][j + 1] - sum[i][j] + a[i][j];
				}
			}
			return sum;
		}
		double[][] sums(double[][] a) {
			double sum[][] = new double[a.length + 1][a[0].length + 1];
			fill(sum, 0);
			for(int i = 0; i < a.length; i ++) {
				for(int j = 0; j < a[i].length; j ++) {
					sum[i + 1][j + 1] = sum[i + 1][j] + sum[i][j + 1] - sum[i][j] + a[i][j];
				}
			}
			return sum;
		}
		long[][] sums(boolean[][] a) {
			long sum[][] = new long[a.length + 1][a[0].length + 1];
			fill(sum, 0);
			for(int i = 0; i < a.length; i ++) {
				for(int j = 0; j < a[i].length; j ++) {
					sum[i + 1][j + 1] = sum[i + 1][j] + sum[i][j + 1] - sum[i][j] + (a[i][j] ? 1 : 0);
				}
			}
			return sum;
		}
		int constrain(int x, int l, int r) { return min(max(x, min(l, r)), max(l, r)); }
		long constrain(long x, long l, long r) { return min(max(x, min(l, r)), max(l, r)); }
		double constrain(double x, double l, double r) { return min(max(x, min(l, r)), max(l, r)); }

		int abs(int x) { return x >= 0 ? x : - x; }
		long abs(long x) { return x >= 0 ? x : - x; }
		double abs(double x) { return x >= 0 ? x : - x; }
		int signum(int x) { return x > 0 ? 1 : x < 0 ? -1 : 0; }
		int signum(long x) { return x > 0 ? 1 : x < 0 ? -1 : 0; }
		int signum(double x) { return x > 0 ? 1 : x < 0 ? -1 : 0; }
		long round(double x) { return Math.round(x); }
		long floor(double x) { return (long)Math.floor(x); }
		int divfloor(int a, int b) { return signum(a) == signum(b) ? a / b : - divceil(abs(a), abs(b)); }
		long divfloor(long a, long b) { return signum(a) == signum(b) ? a / b : - divceil(abs(a), abs(b)); }
		long ceil(double x) { return (long)Math.ceil(x); }
		int divceil(int a, int b) { return a >= 0 && b > 0 ? (a + b - 1) / b
											: a < 0 && b < 0 ? divceil(abs(a), abs(b))
											: - divfloor(abs(a), abs(b)); }
		long divceil(long a, long b) { return a >= 0 && b > 0 ? (a + b - 1) / b
											: a < 0 && b < 0 ? divceil(abs(a), abs(b))
											: - divfloor(abs(a), abs(b)); }
		double sqrt(int x) { return Math.sqrt((double)x); }
		double sqrt(long x) { return Math.sqrt((double)x); }
		double sqrt(double x) { return Math.sqrt(x); }
		long fact(int n) {
			long ans = 1;
			for(int i = 1; i <= n; i ++) ans *= i;
			return ans;
		}
		double pow(double x, double y) { return Math.pow(x, y); }
		long pow(long x, long y) {
			long ans = 1;
			for(; y > 0; y /= 2) {
				if(y % 2 != 0) ans *= x;
				x *= x;
			}
			return ans;
		}
		int gcd(int a, int b) {
			while(true) {
				if(b == 0) return a;
				int tmp = a;
				a = b;
				b = tmp % b;
			}
		}
		long gcd(long a, long b) {
			while(true) {
				if(b == 0) return a;
				long tmp = a;
				a = b;
				b = tmp % b;
			}
		}
		long lcm(long a, long b) { return a / gcd(a, b) * b; }
		int gcd(int[] array) {
			int gcd = 0;
			for(int i = 0; i < array.length; i ++) { gcd = gcd(gcd, array[i]); }
			return gcd;
		}
		long gcd(long[] array) {
			long gcd = 0;
			for(int i = 0; i < array.length; i ++) { gcd = gcd(gcd, array[i]); }
			return gcd;
		}

		long[] mulMat(long[][] mat1, long[] mat2) {
			long mul[] = new long[mat2.length];
			for(int i = 0; i < mul.length; i ++) {
				for(int j = 0; j < mat2.length; j ++) {
					mul[i] += mat1[i][j] * mat2[j];
				}
			}
			return mul;
		}
		long[][] mulMat(long[][] mat1, long[][] mat2) {
			long mul[][] = new long[mat1.length][mat2[0].length];
			for(int i = 0; i < mul.length; i ++) {
				for(int j = 0; j < mul[i].length; j ++) {
					for(int k = 0; k < mat2.length; k ++) {
						mul[i][j] += mat1[i][k] * mat2[k][j];
					}
				}
			}
			return mul;
		}
		long[][] powMat(long[][] mat, long y) {
			int num = mat.length;
			long[][] ans = new long[num][num];
			fill(ans, 0);
			for(int i = 0; i < num; i ++) ans[i][i] = 1;
			long[][] x = new long[num][num];
			for(int i = 0; i < num; i ++) for(int j = 0; j < num; j ++) x[i][j] = mat[i][j];
			for(; y > 0; y /= 2) {
				if(y % 2 != 0) ans = mulMat(ans, x);
				x = mulMat(x, x);
			}
			return ans;
		}

		int upperToInt(char a) { return a - 'A'; }
		int lowerToInt(char a) { return a - 'a'; }
		int numToInt(char a) { return a - '0'; }
		int charToInt(char a) { return a >= 'a' ? lowerToInt(a) : a >= 'A' ? upperToInt(a) : numToInt(a); }
		char intToUpper(int a) { return (char)(a + 'A'); }
		char intToLower(int a) { return (char)(a + 'a'); }
		char intToNum(int a) { return (char)(a + '0'); }
		int[] charToInt(char[] a) {
			int array[] = new int[a.length];
			for(int i = 0; i < a.length; i ++) { array[i] = charToInt(a[i]); }
			return array;
		}

		long[] div(long a) {
			List<Long> divList = new ArrayList<Long>();
			for(long i = 1; i * i <= a; i ++) {
				if(a % i == 0) {
					divList.add(i);
					if(i * i != a) { divList.add(a / i); };
				}
			}

			long div[] = new long[divList.size()];
			for(int i = 0; i < divList.size(); i ++) { div[i] = divList.get(i); }
			return div;
		}

		long[][] factor(long a) {
			List<Long> factorList = new ArrayList<Long>();
			List<Long> degreeList = new ArrayList<Long>();
			for(long i = 2; i * i <= a; i ++) {
				if(a % i == 0) {
					long count = 0;
					while(a % i == 0) {
						a /= i;
						count ++;
					}
					factorList.add(i);
					degreeList.add(count);
				}
			}
			if(a > 1) {
				factorList.add(a);
				degreeList.add(1L);
			}
			long factor[][] = new long[factorList.size()][2];
			for(int i = 0; i < factorList.size(); i ++) {
				factor[i][0] = factorList.get(i);
				factor[i][1] = degreeList.get(i);
			}
			Arrays.sort(factor, (sort1, sort2) -> Long.compare(sort1[0], sort2[0]));
			return factor;
		}

		boolean isPrime(long x) {
			boolean ok = x > 1;
			for(long i = 2; i * i <= x; i ++) {
				ok &= x % i != 0;
				if(!ok) return ok;
			}
			return ok;
		}
		boolean[] prime(int num) {
			boolean prime[] = new boolean[num];
			fill(prime, true);
			prime[0] = false;
			prime[1] = false;
			for(int i = 2; i < num; i ++) {
				if(prime[i]) {
					for(int j = 2; i * j < num; j ++) {
						prime[i * j] = false;
					}
				}
			}
			return prime;
		}

		long[][] countElements(long[] a, boolean sort) {
			int len = a.length;
			long array[] = new long[len];
			for(int i = 0; i < len; i ++) {
				array[i] = a[i];
			}
			if(sort) { Arrays.sort(array); }
			List<Long> elem = new ArrayList<Long>();
			List<Long> cnt = new ArrayList<Long>();
			long tmp = 1;
			for(int i = 1; i <= len; i ++) {
				if(i == len || array[i] != array[i - 1]) {
					elem.add(array[i - 1]);
					cnt.add(tmp);
					tmp = 1;
				}else {
					tmp ++;
				}
			}
			long counts[][] = new long[elem.size()][2];
			for(int i = 0; i < elem.size(); i ++) {
				counts[i][0] = elem.get(i);
				counts[i][1] = cnt.get(i);
			}
			return counts;
		}
		long[][] countElements(String str, boolean sort) {
			int len = str.length();
			char array[] = str.toCharArray();
			if(sort) { Arrays.sort(array); }
			List<Long> elem = new ArrayList<Long>();
			List<Long> cnt = new ArrayList<Long>();
			long tmp = 1;
			for(int i = 1; i <= len; i ++) {
				if(i == len || array[i] != array[i - 1]) {
					elem.add((long)array[i - 1]);
					cnt.add(tmp);
					tmp = 1;
				}else {
					tmp ++;
				}
			}
			long counts[][] = new long[elem.size()][2];
			for(int i = 0; i < elem.size(); i ++) {
				counts[i][0] = elem.get(i);
				counts[i][1] = cnt.get(i);
			}
			return counts;
		}

		int[] baseConvert(long x, int n, int len) {
			int digit[] = new int[len];
			int i = 0;
			long tmp = x;
			while(tmp > 0 && i < len) { digit[i ++] = (int)(tmp % n); tmp /= n; }
			return digit;
		}
		int[] baseConvert(long x, int n) {
			long tmp = x;
			int len = 0;
			while(tmp > 0) { tmp /= n; len ++; }
			return baseConvert(x, n, len);
		}
		int[] baseConvert(int x, int n, int len) {
			int digit[] = new int[len];
			int i = 0;
			int tmp = x;
			while(tmp > 0 && i < len) { digit[i ++] = (int)(tmp % n); tmp /= n; }
			return digit;
		}
		int[] baseConvert(int x, int n) {
			int tmp = x;
			int len = 0;
			while(tmp > 0) { tmp /= n; len ++; }
			return baseConvert(x, n, len);
		}
		long[] baseConvert(long x, long n, int len) {
			long digit[] = new long[len];
			int i = 0;
			long tmp = x;
			while(tmp > 0 && i < len) { digit[i ++] = tmp % n; tmp /= n; }
			return digit;
		}
		long[] baseConvert(long x, long n) {
			long tmp = x;
			int len = 0;
			while(tmp > 0) { tmp /= n; len ++; }
			return baseConvert(x, n, len);
		}

		int numDigits(long a) { return Long.toString(a).length(); }
		long bitFlag(int a) { return 1L << (long)a; }
		boolean isFlagged(long x, int a) { return (x & bitFlag(a)) != 0; }

		long countString(String str, String a) { return (str.length() - str.replace(a, "").length()) / a.length(); }
		long countStringAll(String str, String a) { return str.length() - str.replaceAll(a, "").length(); }

		int lowerBound(int[] array, int key) {
			return BS(array, key, true, true, true);
		}
		int lowerBound(int[] array, int key, int ng, int ok) {
			return BS(array, key, true, true, true, ng, ok);
		}
		int upperBound(int[] array, int key) {
			return BS(array, key, true, true, false);
		}
		int upperBound(int[] array, int key, int ng, int ok) {
			return BS(array, key, true, true, false, ng, ok);
		}
		int cntBS(int[] array, int key, boolean ascending, boolean greater, boolean equals) {
			return BS(array, key, ascending, greater, equals, true);
		}
		int cntBS(int[] array, int key, boolean ascending, boolean greater, boolean equals, int ng, int ok) {
			return BS(array, key, ascending, greater, equals, true, ng, ok);
		}
		int BS(int[] array, int key, boolean ascending, boolean greater, boolean equals) {
			return BS(array, key, ascending, greater, equals, false);
		}
		int BS(int[] array, int key, boolean ascending, boolean greater, boolean equals, int ng, int ok) {
			return BS(array, key, ascending, greater, equals, false, ng, ok);
		}
		int BS(int[] array, int key, boolean ascending, boolean greater, boolean equals, boolean count) {
			int ng = ascending ^ greater ? array.length : -1;
			int ok = ascending ^ greater ? -1 : array.length;
			return BS(array, key, ascending, greater, equals, count, ng, ok);
		}
		int BS(int[] array, int key, boolean ascending, boolean greater, boolean equals, boolean count, int ng, int ok) {
			int index = binarySearch(array, key, greater, equals, ng, ok);
			return count ? (int)abs(ok - index) : index;
		}
		int binarySearch(int[] array, int key, boolean greater, boolean equals, int ng, int ok) {
			while (abs(ok - ng) > 1) {
				int mid = (ok + ng) / 2;
				if(isOKforBinarySearch(array, mid, key, greater, equals)) {
					ok = mid;
				}else {
					ng = mid;
				}
			}
			return ok;
		}
		boolean isOKforBinarySearch(int[] array, int index, int key, boolean greater, boolean equals) {
			return (array[index] > key && greater)
				|| (array[index] < key && !greater)
				|| (array[index] == key && equals);
		}
		int lowerBound(long[] array, long key) {
			return BS(array, key, true, true, true);
		}
		int lowerBound(long[] array, long key, int ng, int ok) {
			return BS(array, key, true, true, true, ng, ok);
		}
		int upperBound(long[] array, long key) {
			return BS(array, key, true, true, false);
		}
		int upperBound(long[] array, long key, int ng, int ok) {
			return BS(array, key, true, true, false, ng, ok);
		}
		int cntBS(long[] array, long key, boolean ascending, boolean greater, boolean equals) {
			return BS(array, key, ascending, greater, equals, true);
		}
		int cntBS(long[] array, long key, boolean ascending, boolean greater, boolean equals, int ng, int ok) {
			return BS(array, key, ascending, greater, equals, true, ng, ok);
		}
		int BS(long[] array, long key, boolean ascending, boolean greater, boolean equals) {
			return BS(array, key, ascending, greater, equals, false);
		}
		int BS(long[] array, long key, boolean ascending, boolean greater, boolean equals, int ng, int ok) {
			return BS(array, key, ascending, greater, equals, false, ng, ok);
		}
		int BS(long[] array, long key, boolean ascending, boolean greater, boolean equals, boolean count) {
			int ng = ascending ^ greater ? array.length : -1;
			int ok = ascending ^ greater ? -1 : array.length;
			return BS(array, key, ascending, greater, equals, count, ng, ok);
		}
		int BS(long[] array, long key, boolean ascending, boolean greater, boolean equals, boolean count, int ng, int ok) {
			int index = binarySearch(array, key, greater, equals, ng, ok);
			return count ? (int)abs(ok - index) : index;
		}
		int binarySearch(long[] array, long key, boolean greater, boolean equals, int ng, int ok) {
			while (abs(ok - ng) > 1) {
				int mid = (ok + ng) / 2;
				if(isOKforBinarySearch(array, mid, key, greater, equals)) {
					ok = mid;
				}else {
					ng = mid;
				}
			}
			return ok;
		}
		boolean isOKforBinarySearch(long[] array, int index, long key, boolean greater, boolean equals) {
			return (array[index] > key && greater)
				|| (array[index] < key && !greater)
				|| (array[index] == key && equals);
		}
		int lowerBound(double[] array, double key) {
			return BS(array, key, true, true, true);
		}
		int lowerBound(double[] array, double key, int ng, int ok) {
			return BS(array, key, true, true, true, ng, ok);
		}
		int upperBound(double[] array, double key) {
			return BS(array, key, true, true, false);
		}
		int upperBound(double[] array, double key, int ng, int ok) {
			return BS(array, key, true, true, false, ng, ok);
		}
		int cntBS(double[] array, double key, boolean ascending, boolean greater, boolean equals) {
			return BS(array, key, ascending, greater, equals, true);
		}
		int cntBS(double[] array, double key, boolean ascending, boolean greater, boolean equals, int ng, int ok) {
			return BS(array, key, ascending, greater, equals, true, ng, ok);
		}
		int BS(double[] array, double key, boolean ascending, boolean greater, boolean equals) {
			return BS(array, key, ascending, greater, equals, false);
		}
		int BS(double[] array, double key, boolean ascending, boolean greater, boolean equals, int ng, int ok) {
			return BS(array, key, ascending, greater, equals, false, ng, ok);
		}
		int BS(double[] array, double key, boolean ascending, boolean greater, boolean equals, boolean count) {
			int ng = ascending ^ greater ? array.length : -1;
			int ok = ascending ^ greater ? -1 : array.length;
			return BS(array, key, ascending, greater, equals, count, ng, ok);
		}
		int BS(double[] array, double key, boolean ascending, boolean greater, boolean equals, boolean count, int ng, int ok) {
			int index = binarySearch(array, key, greater, equals, ng, ok);
			return count ? (int)abs(ok - index) : index;
		}
		int binarySearch(double[] array, double key, boolean greater, boolean equals, int ng, int ok) {
			while (abs(ok - ng) > 1) {
				int mid = (ok + ng) / 2;
				if(isOKforBinarySearch(array, mid, key, greater, equals)) {
					ok = mid;
				}else {
					ng = mid;
				}
			}
			return ok;
		}
		boolean isOKforBinarySearch(double[] array, int index, double key, boolean greater, boolean equals) {
			return (array[index] > key && greater)
				|| (array[index] < key && !greater)
				|| (array[index] == key && equals);
		}
		<T> int lowerBound(T[] array, T key, Comparator<? super T> c) {
			return BS(array, key, true, true, true, c);
		}
		<T> int lowerBound(T[] array, T key, int ng, int ok, Comparator<? super T> c) {
			return BS(array, key, true, true, true, ng, ok, c);
		}
		<T> int upperBound(T[] array, T key, Comparator<? super T> c) {
			return BS(array, key, true, true, false, c);
		}
		<T> int upperBound(T[] array, T key, int ng, int ok, Comparator<? super T> c) {
			return BS(array, key, true, true, false, ng, ok, c);
		}
		<T> int cntBS(T[] array, T key, boolean ascending, boolean greater, boolean equals, Comparator<? super T> c) {
			return BS(array, key, ascending, greater, equals, true, c);
		}
		<T> int cntBS(T[] array, T key, boolean ascending, boolean greater, boolean equals, int ng, int ok, Comparator<? super T> c) {
			return BS(array, key, ascending, greater, equals, true, ng, ok, c);
		}
		<T> int BS(T[] array, T key, boolean ascending, boolean greater, boolean equals, Comparator<? super T> c) {
			return BS(array, key, ascending, greater, equals, false, c);
		}
		<T> int BS(T[] array, T key, boolean ascending, boolean greater, boolean equals, int ng, int ok, Comparator<? super T> c) {
			return BS(array, key, ascending, greater, equals, false, ng, ok, c);
		}
		<T> int BS(T[] array, T key, boolean ascending, boolean greater, boolean equals, boolean count, Comparator<? super T> c) {
			int ng = ascending ^ greater ? array.length : -1;
			int ok = ascending ^ greater ? -1 : array.length;
			return BS(array, key, ascending, greater, equals, count, ng, ok, c);
		}
		<T> int BS(T[] array, T key, boolean ascending, boolean greater, boolean equals, boolean count, int ng, int ok, Comparator<? super T> c) {
			int index = binarySearch(array, key, greater, equals, ng, ok, c);
			return count ? (int)abs(ok - index) : index;
		}
		<T> int binarySearch(T[] array, T key, boolean greater, boolean equals, int ng, int ok, Comparator<? super T> c) {
			while (abs(ok - ng) > 1) {
				int mid = (ok + ng) / 2;
				if(isOKforBinarySearch(array, mid, key, greater, equals, c)) {
					ok = mid;
				}else {
					ng = mid;
				}
			}
			return ok;
		}
		<T> boolean isOKforBinarySearch(T[] array, int index, T key, boolean greater, boolean equals, Comparator<? super T> c) {
			int compare = c.compare(array[index], key);
			return (compare > 0 && greater)
				|| (compare < 0 && !greater)
				|| (compare == 0 && equals);
		}



		void reverse(String[] array) {
			for(int i = 0; i < array.length / 2; i ++) { swap(array, i, array.length - i - 1); }
		}
		void reverse(int[] array) {
			for(int i = 0; i < array.length / 2; i ++) { swap(array, i, array.length - i - 1); }
		}
		void reverse(long[] array) {
			for(int i = 0; i < array.length / 2; i ++) { swap(array, i, array.length - i - 1); }
		}
		void reverse(double[] array) {
			for(int i = 0; i < array.length / 2; i ++) { swap(array, i, array.length - i - 1); }
		}
		void reverse(char[] array) {
			for(int i = 0; i < array.length / 2; i ++) { swap(array, i, array.length - i - 1); }
		}
		void reverse(boolean[] array) {
			for(int i = 0; i < array.length / 2; i ++) { swap(array, i, array.length - i - 1); }
		}
		<T> void reverse(T[] array) {
			for(int i = 0; i < array.length / 2; i ++) { swap(array, i, array.length - i - 1); }
		}
		void fill(int[] array, int x) { Arrays.fill(array, x); }
		void fill(long[] array, long x) { Arrays.fill(array, x); }
		void fill(double[] array, double x) { Arrays.fill(array, x); }
		void fill(char[] array, char x) { Arrays.fill(array, x); }
		void fill(boolean[] array, boolean x) { Arrays.fill(array, x); }
		void fill(int[][] array, int x) { for(int[] a : array) { fill(a, x); } }
		void fill(long[][] array, long x) { for(long[] a : array) { fill(a, x); } }
		void fill(double[][] array, double x) { for(double[] a : array) { fill(a, x); } }
		void fill(char[][] array, char x) { for(char[] a : array) { fill(a, x); } }
		void fill(boolean[][] array, boolean x) { for(boolean[] a : array) { fill(a, x); } }
		void fill(int[][][] array, int x) { for(int[][] a : array) { fill(a, x); } }
		void fill(long[][][] array, long x) { for(long[][] a : array) { fill(a, x); } }
		void fill(double[][][] array, double x) { for(double[][] a : array) { fill(a, x); } }
		void fill(char[][][] array, char x) { for(char[][] a : array) { fill(a, x); } }
		void fill(boolean[][][] array, boolean x) { for(boolean[][] a : array) { fill(a, x); } }

		int[] resize(int[] array, int m, int x) {
			int resized[] = new int[m];
			for(int i = max(0, - x); i < array.length && i + x < m; i ++) { resized[i + x] = array[i]; }
			return resized;
		}
		long[] resize(long[] array, int m, int x) {
			long resized[] = new long[m];
			for(int i = max(0, - x); i < array.length && i + x < m; i ++) { resized[i + x] = array[i]; }
			return resized;
		}
		double[] resize(double[] array, int m, int x) {
			double resized[] = new double[m];
			for(int i = max(0, - x); i < array.length && i + x < m; i ++) { resized[i + x] = array[i]; }
			return resized;
		}
		char[] resize(char[] array, int m, int x) {
			char resized[] = new char[m];
			for(int i = max(0, - x); i < array.length && i + x < m; i ++) { resized[i + x] = array[i]; }
			return resized;
		}
		boolean[] resize(boolean[] array, int m, int x) {
			boolean resized[] = new boolean[m];
			for(int i = max(0, - x); i < array.length && i + x < m; i ++) { resized[i + x] = array[i]; }
			return resized;
		}
		Object[] resize(Object[] array, int m, int x) {
			Object resized[] = new Object[m];
			for(int i = max(0, - x); i < array.length && i + x < m; i ++) { resized[i + x] = array[i]; }
			return resized;
		}

		void shuffleArray(int[] array){
			int n = array.length;
			Random rnd = new Random();
			for(int i = 0; i < n; i ++){
				int tmp = array[i];
				int randomPos = i + rnd.nextInt(n - i);
				array[i] = array[randomPos];
				array[randomPos] = tmp;
			}
		}
		void shuffleArray(long[] array){
			int n = array.length;
			Random rnd = new Random();
			for(int i = 0; i < n; i ++){
				long tmp = array[i];
				int randomPos = i + rnd.nextInt(n - i);
				array[i] = array[randomPos];
				array[randomPos] = tmp;
			}
		}
		void shuffleArray(double[] array){
			int n = array.length;
			Random rnd = new Random();
			for(int i = 0; i < n; i ++){
				double tmp = array[i];
				int randomPos = i + rnd.nextInt(n - i);
				array[i] = array[randomPos];
				array[randomPos] = tmp;
			}
		}

		void swap(String[] array, int i, int j) {
			String tmp = array[i];
			array[i] = array[j];
			array[j] = tmp;
		}
		void swap(int[] array, int i, int j) {
			int tmp = array[i];
			array[i] = array[j];
			array[j] = tmp;
		}
		void swap(long[] array, int i, int j) {
			long tmp = array[i];
			array[i] = array[j];
			array[j] = tmp;
		}
		void swap(double[] array, int i, int j) {
			double tmp = array[i];
			array[i] = array[j];
			array[j] = tmp;
		}
		void swap(char[] array, int i, int j) {
			char tmp = array[i];
			array[i] = array[j];
			array[j] = tmp;
		}
		void swap(boolean[] array, int i, int j) {
			boolean tmp = array[i];
			array[i] = array[j];
			array[j] = tmp;
		}
		<T> void swap(T[] array, int i, int j) {
			T tmp = array[i];
			array[i] = array[j];
			array[j] = tmp;
		}

		int[] compress(int[] a) {
			int num = a.length;
			Set<Integer> ss = new TreeSet<>();
			for(int i = 0; i < num; i ++) ss.add(a[i]);
			int compressed[] = new int[ss.size()];
			int j = 0;
			for(Integer x : ss) compressed[j ++] = x;
			for(int i = 0; i < num; i ++) a[i] = lowerBound(compressed, a[i]);
			return compressed;
		}
		long[] compress(long[] a) {
			int num = a.length;
			Set<Long> ss = new TreeSet<>();
			for(int i = 0; i < num; i ++) ss.add(a[i]);
			long compressed[] = new long[ss.size()];
			int j = 0;
			for(Long x : ss) compressed[j ++] = x;
			for(int i = 0; i < num; i ++) a[i] = lowerBound(compressed, a[i]);
			return compressed;
		}
		double[] compress(double[] a) {
			int num = a.length;
			Set<Double> ss = new TreeSet<>();
			for(int i = 0; i < num; i ++) ss.add(a[i]);
			double compressed[] = new double[ss.size()];
			int j = 0;
			for(Double x : ss) compressed[j ++] = x;
			for(int i = 0; i < num; i ++) a[i] = lowerBound(compressed, a[i]);
			return compressed;
		}

		long INF = (long)1e18 + 7;
		boolean isINF(long a) { return abs(a) > INF / 1000; }
		boolean isPlusINF(long a) { return a > 0 && isINF(a); }
		boolean isMinusINF(long a) { return isPlusINF(- a); }
		int I_INF = (int)1e9 + 7;
		boolean isINF(int a) { return abs(a) > I_INF / 1000; }
		boolean isPlusINF(int a) { return a > 0 && isINF(a); }
		boolean isMinusINF(int a) { return isPlusINF(- a); }


		// mods
		final long MOD = (long)1e9 + 7; // 998244353;
		public long mod(long i) { i %= MOD; return i + (i < 0 ? MOD : 0); }

		long pow_m(long x, long y) {
			long ans = 1;
			for(; y > 0; y /= 2) {
				if(y % 2 != 0) ans = mod(ans * x);
				x = mod(x * x);
			}
			return ans;
		}
		long[] pows_m(long x, int max) {
			long pow[] = new long[max + 1];
			pow[0] = 1;
			for(int i = 0; i < max; i ++) {
				pow[i + 1] = mod(pow[i] * x);
			}
			return pow;
		}
		long fact_m(int n) {
			long ans = 1;
			for(int i = 1; i <= n; i ++) ans = mod(ans * i);
			return ans;
		}

		int MAX_INV_SIZE = 100_100;
		HashMap<Long, Long> invMap = new HashMap<>();
		long inv(long x) {
			x = mod(x);
			if(invMap.containsKey(x)) { return invMap.get(x); }
			if(invMap.size() >= MAX_INV_SIZE) { return calInv(x); }
			invMap.put(x, calInv(x));
			return invMap.get(x);
		}
		long calInv(long x) { return pow_m(x, MOD - 2); }

		int MAX_FACT = 5_000_100;
		long fact[];
		long invFact[];
		boolean isFactPrepared = false;
		HashMap<Integer, long[]> factMap;
		void prepareFact() {
			fact = new long[MAX_FACT];
			Arrays.fill(fact, 0);
			invFact = new long[MAX_FACT];
			Arrays.fill(invFact, 0);
			fact[0] = 1;
			int maxIndex = min(MAX_FACT, (int)MOD);
			for(int i = 1; i < maxIndex; i ++) { fact[i] = mod(fact[i - 1] * i); }
			invFact[maxIndex - 1] = inv(fact[maxIndex - 1]);
			for(int i = maxIndex - 1; i > 0; i --) { invFact[i - 1] = mod(invFact[i] * i); }

			factMap = new HashMap<>();
			isFactPrepared = true;
		}

		long P(int n, int r) {
			if(!isFactPrepared) { prepareFact(); }
			if(n < 0 || r < 0 || n < r) { return 0; }
			if(n >= MAX_FACT) {
				if(!factMap.containsKey(n)) {
					long largeFact[] = new long[MAX_FACT];
					factMap.put(n, largeFact);
					fill(largeFact, -INF);
					largeFact[0] = 1;
				}
				long largeFact[] = factMap.get(n);
				int i = r;
				while(isINF(largeFact[i])) { i --; }
				for(; i < r; i ++) { largeFact[i + 1] = mod(largeFact[i] * (n - i)); }
				return largeFact[r];
			}
			return mod(fact[n] * invFact[n - r]);
		}
		long C(int n, int r) {
			if(!isFactPrepared) { prepareFact(); }
			if(n < 0 || r < 0 || n < r) { return 0; }
			return mod(P(n, r) * invFact[r]);
		}
		long H(int n, int r) { return C((n - 1) + r, r); }


		// grid
		class Grids {
			int h;
			int w;
			Grid[][] gs;
			Grid[] gi;
			Grids(int h, int w) {
				this.h = h;
				this.w = w;
				gs = new Grid[h][w];
				gi = new Grid[h * w];
				for(int i = 0; i < h; i ++) {
					for(int j = 0; j < w; j ++) {
						gs[i][j] = new Grid(i, j, h, w);
						gi[gs[i][j].i] = gs[i][j];
					}
				}
			}

			void init(boolean[][] b) {
				for(int i = 0; i < h; i ++) {
					for(int j = 0; j < w; j ++) {
						gs[i][j].b = b[i][j];
					}
				}
			}
			void init(long[][] val) {
				for(int i = 0; i < h; i ++) {
					for(int j = 0; j < w; j ++) {
						gs[i][j].val = val[i][j];
					}
				}
			}

			Grid get(int x, int y) { return isValid(x, y, h, w) ? gs[x][y] : null; }

			int dx[] = {0, -1, 1, 0, 0, -1, 1, -1, 1};
			int dy[] = {0, 0, 0, -1, 1, -1, -1, 1, 1};
			Grid next(int x, int y, int i) { return next(gs[x][y], i); }
			Grid next(Grid g, int i) {
				return isValid(g.x + dx[i], g.y + dy[i], g.h, g.w)
					? gs[g.x + dx[i]][g.y + dy[i]]
					: null;
			}
		}
		class Grid implements Comparable<Grid> {
			int x;
			int y;
			int h;
			int w;
			int i;
			boolean b;
			long val;

			Grid() {  }
			Grid(int x, int y, int h, int w) { init(x, y, h, w, false, 0); }
			Grid(int x, int y, int h, int w, boolean b) { init(x, y, h, w, b, 0); }
			Grid(int x, int y, int h, int w, long val) { init(x, y, h, w, false, val); }
			Grid(int x, int y, int h, int w, boolean b, long val) { init(x, y, h, w, b, val); }

			void init(int x, int y, int h, int w, boolean b, long val) {
				this.x = x;
				this.y = y;
				this.h = h;
				this.w = w;
				this.b = b;
				this.val = val;
				i = x * w + y;
			}

			@Override
			public int compareTo(Grid g) {
				return Long.compare(this.val, g.val);
			}
		}

		boolean isValid(int x, int y, int h, int w) {
			return x >= 0 && x < h && y >= 0 && y < w;
		}
		boolean isValid(Grid g) {
			return isValid(g.x, g.y, g.h, g.w);
		}

		// graph
		class Graph {
			int numNode;
			int numEdge;
			boolean directed;
			Edge edges[];
			Node nodes[];
			Node reversedNodes[];

			Graph(int numNode, int numEdge, boolean directed) {
				this.numNode = numNode;
				this.numEdge = numEdge;
				this.directed = directed;
				nodes = new Node[numNode];
				reversedNodes = new Node[numNode];
				for(int i = 0; i < numNode; i ++) {
					nodes[i] = new Node(i);
					reversedNodes[i] = new Node(i);
				}
			}

			void init(Edge[] edges) {
				this.edges = edges;
				for(Edge edge : edges) {
					nodes[edge.source].add(edge.target, edge.cost);
					if(directed) {
						reversedNodes[edge.target].add(edge.source, edge.cost);
					}else {
						nodes[edge.target].add(edge.source, edge.cost);
					}
				}
			}

			void clearNodes() {
				for(Node n : nodes) { n.clear(); }
				for(Node n : reversedNodes) { n.clear(); }
			}
		}

		class Node extends ArrayList<Edge> {
			int id;

			Node(int id) {
				this.id = id;
			}
			void add(int target, long cost) {
				add(new Edge(id, target, cost));
			}
		}

		class Edge implements Comparable<Edge> {
			int source;
			int target;
			long cost;
			Edge(int source, int target, long cost) {
				this.source = source;
				this.target = target;
				this.cost = cost;
			}

			@Override
			public int compareTo(Edge e) {
				return Long.compare(this.cost, e.cost);
			}
		}

public void solve() {
	int n = ni();
	int k = ni();
	prtln(divceil(n - 1, k - 1));
}



	}
}