#include <iostream>
#include <algorithm>

using namespace std;

long long c,n;

bool cmp(int x,int y) {return x < y;}

int main(){
    cin >> c >> n;
    long long a[n];
    
    
    for(int i = 0;i < n;i++) cin >> a[i];
    
    sort(a+0,a+n,cmp);
    
    int sum = 0;
    for(int i = n;i > n/2;i--){
        for(int j = 0;j < n/2;j++){  
            if(a[j] - a[i] == c) sum++;
        }
    }
	cout << sum;
	return 0;
}