C. Annoying Present
time limit per test
2 seconds
memory limit per test
256 megabytes
input
standard input
output
standard output

Alice got an array of length nn 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 mm changes of the following form. For some integer numbers xx and dd, he chooses an arbitrary position ii (1≤i≤n1≤i≤n) and for every j∈[1,n]j∈[1,n] adds x+d⋅dist(i,j)x+d⋅dist(i,j) to the value of the jj-th cell. dist(i,j)dist(i,j) is the distance between positions ii and jj (i.e. dist(i,j)=|i−j|dist(i,j)=|i−j|, where |x||x| is an absolute value of xx).

For example, if Alice currently has an array [2,1,2,2][2,1,2,2] and Bob chooses position 33 for x=−1x=−1 and d=2d=2 then the array will become [2−1+2⋅2, 1−1+2⋅1, 2−1+2⋅0, 2−1+2⋅1][2−1+2⋅2, 1−1+2⋅1, 2−1+2⋅0, 2−1+2⋅1] = [5,2,1,3][5,2,1,3]. Note that Bob can't choose position ii outside of the array (that is, smaller than 11 or greater than nn).

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 nn and mm (1≤n,m≤1051≤n,m≤105) — the number of elements of the array and the number of changes.

Each of the next mm lines contains two integers xixi and didi (−103≤xi,di≤103−103≤xi,di≤103) — the parameters for the ii-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 10−610−6.

Examples
input
Copy 
2 3
-1 3
0 0
-1 -4
output
Copy 
-2.500000000000000
input
Copy 
3 2
0 2
5 0
output
Copy 
7.000000000000000

题意：i 是在[1 n]之间的任意一个数，

sum = j + x[i] + d[i] * | j - i |;

j每次都被更新上一个sum的值，x[i] , d[i]都是确定不变的，那么就剩|j-i|的值了

例如：1  2  3  4  

           0  1  2  3   

           1  0  1  2  

           2  1  0  1 

           3  2  1  0

    最大的是0+1 + 2+ 3 -----有个公式 sum = n*(n+1) / 2;

    最小的是中间 1+1+2+0-----也可以利用这个公式

sum += n * x[i] + d[i]*(k1+k2+k3...+kn)

最后答案为sum/n.

代码：

#include <stdio.h>
#include <iostream>
using namespace std;
const int maxn = 100005;
long long int init(long long int n)
{
	return n*(n+1)/2;
	
}
int main()
{
	long long int n , m , x[maxn] , d[maxn] , s , retmax , retmin;
	long long int sum = 0;
	cin >> n >> m;
	for(int i = 0 ; i < m ; i++)
	{
		cin >> x[i] >> d[i];
	}
		retmin = init((n-1)/2) * 2;	
		if(n%2==0)
	{
		 retmin += n/2;;
	}

	retmax = init(n-1);
	
	for(int i = 0 ; i < m ; i++)
	{
		sum += n * x[i] ;
		if(d[i] >= 0)
		{
			sum += d[i] * retmax;
		}
		else
		{
		sum += d[i] * retmin;
		}
	}
	double qw = (double)sum/n;
	printf("%.15f\n" , qw);
	return 0;
 }