Problem: Alice got an array of length n as a birthday present once again! This is the third year in a row! 

And what is more disappointing, it is overwhelmengly boring, filled entirely with zeros. Bob decided to apply some changes to the array to cheer up Alice.

Bob has chosen m changes of the following form. For some integer numbers x and d, he chooses an arbitrary position i (1 ≤ i ≤ n) and for every j ∈ [1, n] adds x + d ⋅ dist(i, j) to the value of the j-th cell. dist(i, j) is the distance between positions i and j (i.e. dist(i, j) = |i - j|, where |x| is an absolute value of x).

For example, if Alice currently has an array [2, 1, 2, 2] and Bob chooses position 3 for x = -1 and d = 2 then the array will become [2 - 1 + 2 ⋅ 2,~1 - 1 + 2 ⋅ 1,~2 - 1 + 2 ⋅ 0,~2 - 1 + 2 ⋅ 1] = [5, 2, 1, 3]. Note that Bob can't choose position i outside of the array (that is, smaller than 1 or greater than n).

Alice will be the happiest when the elements of the array are as big as possible. Bob claimed that the arithmetic mean value of the elements will work fine as a metric.

What is the maximum arithmetic mean value Bob can achieve?

Input

The first line contains two integers n and m (1 ≤ n, m ≤ 10^5) — the number of elements of the array and the number of changes.

Each of the next m lines contains two integers x_i and d_i (-10^3 ≤ x_i, d_i ≤ 10^3) — the parameters for the i-th change.

Output

Print the maximal average arithmetic mean of the elements Bob can achieve.

Your answer is considered correct if its absolute or relative error doesn't exceed 10^{-6}.

Examples

Input

2 3
-1 3
0 0
-1 -4


Output

-2.500000000000000


Input

3 2
0 2
5 0


Output

7.000000000000000

Solution (in JAVA):

Solution:
import java.util.*;
import java.awt.List;
import java.io.*;
import java.lang.*;
import java.lang.reflect.Array;

public class code3 {

	
	
	public static void main(String[] args) {
		InputReader in = new InputReader(System.in);
		PrintWriter pw = new PrintWriter(System.out);

		// Code starts..

		

		int n = in.nextInt();
		int m = in.nextInt();
		
		long[] c = new long[n];
		long sum = 0;
		long t = n;
		for(int i=0; i<m; i++)
		{
			sum += in.nextLong()*(long)n;
			long d = in.nextLong();
			if(d>0)
			{
				long tmp = ((long)n*(long)(n-1))/(long)2;
				sum += tmp*d;
			}
			else
			{
				if(n%2==0)
				{
					long n1 =  n/2;
					long tmp = ((long)n1*(long)(n1+1))/(long)2 + ((long)n1*(long)(n1-1))/(long)2;
					sum += tmp*d;
				}
				else
				{
					long n1 = n/2;
					long tmp = ((long)n1*(long)(n1+1));
					sum += tmp*d;
			
				}
				
			}
			
		}
		
		pw.println((double)sum/t);
		
		
		
		// code ends...

		pw.flush();
		pw.close();
	}

	
	
	
	    
	
	
	
	public static long[][] mul(long[][] a, long[][] b){
		
		
		int n = a.length;
		int m = b[0].length;
		long[][] c = new long[n][m];
		
		for(int i=0; i<n; i++)
			for(int j=0; j<m; j++)
			{
				long tmp = 0;
				for(int k=0; k<a[0].length; k++)
					tmp =( (tmp+mod)%mod+(a[i][k]*b[k][j]+mod)%mod)%mod;
				c[i][j] = (tmp+mod)%mod;
			}
		
		return c;
	}
	
	
	
	public static long[][] pow(long[][] a,int  n)
	{
		
		long[][] res = a;
		while(n>0)
		{
			if(n%2==1)
			{
				res = mul(a, res);
			}
			
			a = mul(a, a);
			
			n/=2;
			
			
		}
		
		return res;
		
		
	}
	
	
	
	
	

	
	
	

	
	public static boolean isSubSequence(char[] str2, String str1) {

		int j = 0;
		int m = str1.length();
		int n = str2.length;

		for (int i = 0; i < n && j < m; i++)

			if (str1.charAt(j) == str2[i])

				j++;

		return (j == m);
	}

	static class InputReader {

		private final InputStream stream;
		private final byte[] buf = new byte[8192];
		private int curChar, snumChars;
		private SpaceCharFilter filter;

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

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

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

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

		public int[] nextIntArray(int n) {
			int a[] = new int[n];
			for (int i = 0; i < n; i++) {
				a[i] = nextInt();
			}
			return a;
		}

		public String readString() {
			int c = snext();
			while (isSpaceChar(c)) {
				c = snext();
			}
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = snext();
			} while (!isSpaceChar(c));
			return res.toString();
		}

		public String nextLine() {
			int c = snext();
			while (isSpaceChar(c))
				c = snext();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = snext();
			} while (!isEndOfLine(c));
			return res.toString();
		}

		public boolean isSpaceChar(int c) {
			if (filter != null)
				return filter.isSpaceChar(c);
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}

		private boolean isEndOfLine(int c) {
			return c == '\n' || c == '\r' || c == -1;
		}

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

	public static long c = 0;

	public static long mod = (long)Math.pow(10, 6)+3;
	public static int d;
	public static int p;
	public static int q;
	public static boolean flag;
	//public static long INF = Long.MAX_VALUE;

	public static void factSieve(int[] fact, long n) {

		for (int i = 2; i <= n; i += 2)
			fact[i] = 2;

		for (int i = 3; i <= n; i += 2) {
			if (fact[i] == 0) {
				fact[i] = i;

				for (int j = i; (long) j * i <= n; j++) {
					fact[(int) ((long) i * j)] = i;
				}
			}
		}

		/*
		 * int k = 1000; while(k!=1) { System.out.print(a[k]+" "); k /= a[k];
		 * 
		 * }
		 */
	}

	public static long floorsqrt(long n) {
		long ans = 0;
		long l = 0;
		long r = n;
		long mid = 0;
		while (l <= r) {
			mid = (l + r) / 2;
			if (mid * mid == n)
				return mid;

			if (mid * mid > n) {
				r = mid - 1;
			} else {
				ans = mid;
				l = mid + 1;
			}

		}

		return ans;
	}

	public static long fun(int[] a, int[] b, int m, int n) {
		long result = 0;
		for (int i = 0; i < m; i++)
			for (int j = 0; j < m; j++) {
				long[] fib = new long[Math.max(2, n + 2)];
				fib[1] = a[i];
				fib[2] = b[j];
				for (int k = 3; k <= n; k++)
					fib[k] = (fib[k - 1] % mod + fib[k - 2] % mod) % mod;
				result = (result % mod + fib[n] % mod) % mod;
			}

		return result;

	}

	/*
	 * public static double slope(pair p1, pair p2) { double m = INF; if((p1.x -
	 * p2.x)!=0) m = p1.y - p2.y/(p1.x - p2.x);
	 * 
	 * 
	 * return Math.abs(m);
	 * 
	 * }
	 */

	public static int gcd(int p2, int p22) {
		if (p2 == 0)
			return (int) p22;
		return gcd(p22 % p2, p2);
	}

	public static int findGCD(int arr[], int n) {
		int result = arr[0];
		for (int i = 1; i < n; i++)
			result = gcd(arr[i], result);

		return result;
	}

	public static void nextGreater(long[] a, int[] ans) {

		Stack<Integer> stk = new Stack<>();
		stk.push(0);

		for (int i = 1; i < a.length; i++) {

			if (!stk.isEmpty()) {
				int s = stk.pop();
				while (a[s] < a[i]) {
					ans[s] = i;
					if (!stk.isEmpty())
						s = stk.pop();
					else
						break;
				}
				if (a[s] >= a[i])
					stk.push(s);
			}

			stk.push(i);

		}
		return;

	}

	public static void nextGreaterRev(long[] a, int[] ans) {

		int n = a.length;
		int[] pans = new int[n];
		Arrays.fill(pans, -1);
		long[] arev = new long[n];
		for (int i = 0; i < n; i++)
			arev[i] = a[n - 1 - i];

		Stack<Integer> stk = new Stack<>();
		stk.push(0);

		for (int i = 1; i < n; i++) {

			if (!stk.isEmpty()) {
				int s = stk.pop();
				while (arev[s] < arev[i]) {
					pans[s] = n - i - 1;
					if (!stk.isEmpty())
						s = stk.pop();
					else
						break;
				}
				if (arev[s] >= arev[i])
					stk.push(s);
			}

			stk.push(i);

		}

		// for(int i=0; i<n; i++)
		// System.out.print(pans[i]+" ");

		for (int i = 0; i < n; i++)
			ans[i] = pans[n - i - 1];

		return;

	}


	public static void nextSmallerRev(int[] a, int[] ans) {

		int n = a.length;
		int[] pans = new int[n];
		Arrays.fill(pans, -1);
		long[] arev = new long[n];
		for (int i = 0; i < n; i++)
			arev[i] = a[n - 1 - i];

		Stack<Integer> stk = new Stack<>();
		stk.push(0);

		for (int i = 1; i < n; i++) {

			if (!stk.isEmpty()) {
				int s = stk.pop();
				while (arev[s] > arev[i]) {
					pans[s] = n - i - 1;
					if (!stk.isEmpty())
						s = stk.pop();
					else
						break;
				}
				if (arev[s] <= arev[i])
					stk.push(s);
			}

			stk.push(i);

		}

		// for(int i=0; i<n; i++)
		// System.out.print(pans[i]+" ");

		for (int i = 0; i < n; i++)
			ans[i] = pans[n - i - 1];

		return;

	}

	
	public static void nextSmaller(int[] a, int[] ans) {

		Stack<Integer> stk = new Stack<>();
		stk.push(0);

		for (int i = 1; i < a.length; i++) {

			if (!stk.isEmpty()) {
				int s = stk.pop();
				while (a[s] > a[i]) {
					ans[s] = i;
					if (!stk.isEmpty())
						s = stk.pop();
					else
						break;
				}
				if (a[s] <= a[i])
					stk.push(s);
			}

			stk.push(i);

		}
		return;

	}

	public static long lcm(int[] numbers) {
		long lcm = 1;
		int divisor = 2;
		while (true) {
			int cnt = 0;
			boolean divisible = false;
			for (int i = 0; i < numbers.length; i++) {
				if (numbers[i] == 0) {
					return 0;
				} else if (numbers[i] < 0) {
					numbers[i] = numbers[i] * (-1);
				}
				if (numbers[i] == 1) {
					cnt++;
				}
				if (numbers[i] % divisor == 0) {
					divisible = true;
					numbers[i] = numbers[i] / divisor;
				}
			}
			if (divisible) {
				lcm = lcm * divisor;
			} else {
				divisor++;
			}
			if (cnt == numbers.length) {
				return lcm;
			}
		}
	}

	public static long fact(long n) {

		long factorial = 1;
		for (int i = 1; i <= n; i++) {
			factorial *= i;
		}
		return factorial;
	}

	public static int lowerLimit(int[] a, int n) {
		int ans = 0;

		int ll = 0;
		int rl = a.length - 1;
		// System.out.println(a[rl]+" "+n);
		if (a[0] > n)
			return 0;
		if (a[0] == n)
			return 1;
		else if (a[rl] <= n)
			return rl + 1;

		while (ll <= rl) {

			int mid = (ll + rl) / 2;
			if (a[mid] == n) {
				ans = mid + 1;
				break;
			}

			else if (a[mid] > n) {
				rl = mid - 1;

			} else {
				ans = mid + 1;
				ll = mid + 1;
			}
		}

		return ans;
	}

	public static long choose(long total, long choose) {
		if (total < choose)
			return 0;
		if (choose == 0 || choose == total)
			return 1;
		return (choose(total - 1, choose - 1) + choose(total - 1, choose)) % mod;
	}

	public static int[] suffle(int[] a, Random gen) {
		int n = a.length;
		for (int i = 0; i < n; i++) {
			int ind = gen.nextInt(n - i) + i;
			int temp = a[ind];
			a[ind] = a[i];
			a[i] = temp;
		}
		return a;
	}

	public static int floorSearch(int arr[], int low, int high, int x) {
		if (low > high)
			return -1;

		if (x > arr[high])
			return high;
		int mid = (low + high) / 2;

		if (mid > 0 && arr[mid - 1] < x && x < arr[mid])
			return mid - 1;

		if (x < arr[mid])
			return floorSearch(arr, low, mid - 1, x);

		return floorSearch(arr, mid + 1, high, x);
	}

	public static void swap(int a, int b) {
		int temp = a;
		a = b;
		b = temp;
	}

	public static ArrayList<Integer> primeFactorization(int n) {
		ArrayList<Integer> a = new ArrayList<Integer>();
		for (int i = 2; i * i <= n; i++) {
			while (n % i == 0) {
				a.add(i);
				n /= i;
			}
		}
		if (n != 1)
			a.add(n);
		return a;
	}
	
	
	

	public static void sieve(boolean[] isPrime, int n) {
		for (int i = 1; i < n; i++)
			isPrime[i] = true;

		isPrime[0] = false;
		isPrime[1] = false;

		for (int i = 2; i * i < n; i++) {
			if (isPrime[i] == true) {
				for (int j = (2 * i); j < n; j += i)
					isPrime[j] = false;
			}
		}
	}

	public static int lowerbound(ArrayList<Long> net, long c2) {
		int i = Collections.binarySearch(net, c2);
		if (i < 0)
			i = -(i + 2);
		return i;

	}
	
	public static int lowerboundArray(int[] dis, int c2) {
		int i = Arrays.binarySearch(dis, c2);
		if (i < 0)
			i = -(i + 2);
		return i;

	}

	public static int uperbound(ArrayList<Integer> list, int c2) {
		int i = Collections.binarySearch(list, c2);
		if (i < 0)
			i = -(i + 1);
		return i;

	}

	public static int uperboundArray(int[] dis, int c2) {
		int i = Arrays.binarySearch(dis, c2);
		if (i < 0)
			i = -(i + 1);
		return i;

	}

	public static long[] sort(long[] a) {
		Random gen = new Random();
		int n = a.length;
		for (int i = 0; i < n; i++) {
			int ind = gen.nextInt(n - i) + i;
			long temp = a[ind];
			a[ind] = a[i];
			a[i] = temp;
		}

		Arrays.sort(a);
		return a;
	}

	public static int[] sort(int[] a) {
		Random gen = new Random();
		int n = a.length;
		for (int i = 0; i < n; i++) {
			int ind = gen.nextInt(n - i) + i;
			int temp = a[ind];
			a[ind] = a[i];
			a[i] = temp;
		}

		Arrays.sort(a);
		return a;
	}

	public static int GCD(int a, int b) {
		if (b == 0)
			return a;
		else
			return GCD(b, a % b);
	}

	public static long GCD(long a, long b) {
		if (b == 0)
			return a;
		else
			return GCD(b, a % b);
	}

	public static void extendedEuclid(int A, int B) {
		if (B == 0) {
			d = A;
			p = 1;
			q = 0;
		} else {
			extendedEuclid(B, A % B);
			int temp = p;
			p = q;
			q = temp - (A / B) * q;
		}
	}

	public static long LCM(long a, long b) {
		return (a * b) / GCD(a, b);
	}

	public static int LCM(int a, int b) {
		return (a * b) / GCD(a, b);
	}

	public static int binaryExponentiation(int x, int n) {
		int result = 1;
		while (n > 0) {
			if (n % 2 == 1)
				result = result * x;
			x = x * x;
			n = n / 2;
		}
		return result;
	}

	public static int[] countDer(int n) {
		int der[] = new int[n + 1];

		der[0] = 1;
		der[1] = 0;
		der[2] = 1;

		for (int i = 3; i <= n; ++i)
			der[i] = (i - 1) * (der[i - 1] + der[i - 2]);

		// Return result for n
		return der;
	}

	static long binomialCoeff(int n, int k) {
		long C[][] = new long[n + 1][k + 1];
		int i, j;

		// Calculate value of Binomial Coefficient in bottom up manner
		for (i = 0; i <= n; i++) {
			for (j = 0; j <= Math.min(i, k); j++) {
				// Base Cases
				if (j == 0 || j == i)
					C[i][j] = 1;

				// Calculate value using previosly stored values
				else
					C[i][j] = C[i - 1][j - 1] + C[i - 1][j];
			}
		}

		return C[n][k];
	}

	public static long binaryExponentiation(long x, long n) {
		long result = 1;
		while (n > 0) {
			if (n % 2 == 1)
				result = result * x;
			x = (x % mod * x % mod) % mod;
			n = n / 2;
		}
		return result;
	}

	public static int modularExponentiation(int x, int n, int M) {
		int result = 1;
		while (n > 0) {
			if (n % 2 == 1)
				result = (result * x) % M;
			x = (x * x) % M;
			n = n / 2;
		}
		return result;
	}

	public static long modularExponentiation(long x, long n, long M) {
		long result = 1;
		while (n > 0) {
			if (n % 2 == 1)
				result = (result * x) % M;
			x = (x * x) % M;
			n = n / 2;
		}
		return result;
	}

	public static int modInverse(int A, int M) {
		return modularExponentiation(A, M - 2, M);
	}

	public static long sie(long A, long M) {
		return modularExponentiation(A, M - 2, M);
	}

	public static boolean checkYear(int year) {
		if (year % 400 == 0)
			return true;

		if (year % 100 == 0)
			return false;

		if (year % 4 == 0)
			return true;
		return false;
	}

	public static boolean isPrime(int n) {

		if (n <= 1)
			return false;
		if (n <= 3)
			return true;

		if (n % 2 == 0 || n % 3 == 0)
			return false;

		for (int i = 5; i * i <= n; i = i + 6) {
			if (n % i == 0 || n % (i + 2) == 0)
				return false;
		}

		return true;
	}

	static class pair implements Comparable<pair>

	{
		Long x, y;

		pair(long x, long y) {
			this.x = x;
			this.y = y;
		}

		public int compareTo(pair o) {
			int result = x.compareTo(o.x);
			if (result == 0)
				result = y.compareTo(o.y);

			return result;
		}

		public String toString() {
			return x + " " + y;
		}

		public boolean equals(Object o) {
			if (o instanceof pair) {
				pair p = (pair) o;
				if(p.x-x==0 && p.y-y==0)
					return true;
				else
				return false;
			}
			return false;
		}

		public int hashCode() {
			return new Long(x).hashCode() * 31 + new Long(y).hashCode();
		}

	}
	

	static class triplet implements Comparable<triplet> {
		Integer x, y, z;

		triplet(int x, int y, int z) {
			this.x = x;
			this.y = y;
			this.z = z;
		}

		public int compareTo(triplet o) {
			int result = x.compareTo(o.x);
			if (result == 0)
				result = y.compareTo(o.y);
			if (result == 0)
				result = z.compareTo(o.z);

			return result;
		}

		public boolean equlas(Object o) {
			if (o instanceof triplet) {
				triplet p = (triplet) o;
				return x == p.x && y == p.y && z == p.z;
			}
			return false;
		}

		public String toString() {
			return x + " " + y + " " + z;
		}

		public int hashCode() {
			return new Long(x).hashCode() * 31 + new Long(y).hashCode() + new Long(z).hashCode();
		}
	}

	
	static class query implements Comparable<query> {
		Integer l,r, x, idx, tp;

	
		query(int l, int r, int x, int id, int tp)
		{
			this.l = l ;
			this.r = r;
			this.x = x;
			this.idx = id;
			this.tp = tp;
		}
		
		public int compareTo(query o) {
			int result = x.compareTo(o.x);
			
			return result;
		}

		
	

		
	}
	
	/*
	 * static class node implements Comparable<node>
	 * 
	 * { Integer x, y, z; node(int x,int y, int z) { this.x=x; this.y=y; this.z=z; }
	 * 
	 * public int compareTo(pair o) { int result = x.compareTo(o.x); if(result==0)
	 * result = y.compareTo(o.y); if(result==0) result = z.compareTo(z); return
	 * result; }
	 * 
	 * @Override public int compareTo(node o) { // TODO Auto-generated method stub
	 * return 0; } }
	 */
}