import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.BufferedWriter;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.util.InputMismatchException;
import java.io.IOException;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author L_aka_ryuga
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        OutputWriter out = new OutputWriter(outputStream);
        CMinimization solver = new CMinimization();
        solver.solve(1, in, out);
        out.close();
    }

    static class CMinimization {
        public void solve(int testNumber, InputReader in, OutputWriter out) {
        /*
        * Provided it is permutation thus each element will occour once
        * so check position of 1 as it is minimum and then find movements required
        * testcases;-
        * testcase1:-
4 2
1 2 3 4
answer:-
3

testcase2:-
4 3
1 2 3 4
answer:-
2

testcase3:-
4 4
1 2 3 4
answer:-
1

testcase 4:-
4 2
3 2 1 4
answer:-
3

testcases 5:-
4 3
3 2 1 4
answer:-
2

testcase 6:-
4 4
3 2 1 4
answer:
1

testcase 7:-
4 2
4 3 2 1
answer
3

testcase 8:-
4 3
4 3 2 1
answer
2

testcase 9
4 4
4 3 2 1
answer
1
        * */
            int n = in.ni(), k = in.ni();
            int[] arr = in.nia(n);
            out.println((n - 1) % (k - 1) == 0 ? (n - 1) / (k - 1) : (n - 1) / (k - 1) + 1);


        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private InputReader.SpaceCharFilter filter;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public int read() {
            if (numChars == -1) {
                throw new InputMismatchException();
            }
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0) {
                    return -1;
                }
            }
            return buf[curChar++];
        }

        public int ni() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public boolean isSpaceChar(int c) {
            if (filter != null) {
                return filter.isSpaceChar(c);
            }
            return isWhitespace(c);
        }

        public static boolean isWhitespace(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public int[] nia(int n) {
            int[] array = new int[n];
            for (int i = 0; i < n; ++i) array[i] = ni();
            return array;
        }

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);

        }

    }

    static class OutputWriter {
        private final PrintWriter writer;

        public OutputWriter(OutputStream outputStream) {
            writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
        }

        public OutputWriter(Writer writer) {
            this.writer = new PrintWriter(writer);
        }

        public void close() {
            writer.close();
        }

        public void println(int i) {
            writer.println(i);
        }

    }
}



Measured memory_bytes: 21844