#include<stdio.h>

#define NMAX 9007

int Ap[NMAX];
int a[NMAX];
int s = 0, n;

int main(){
    freopen("medie.in", "r", stdin);
    freopen("medie.out", "w", stdout);
    scanf("%d", &n);
    for(int i = 1; i <= n; ++ i)
        scanf("%d", &a[i]);
    for(int i = 1; i <= n; ++ i)
        for(int j = i + 1; j <= n; ++ j){
            int Sum = a[i] + a[j];
            if(Sum % 2 == 0)
                ++ Ap[Sum / 2];
            if(a[i] == a[j])
                s -= 2;
        }
    for(int i = 1; i <= n; ++ i)
        s += Ap[a[i]];
    printf("%d", s);
    return 0;
}