#include <cstdio>

using namespace std;

int v[9005];
int frecv[14005];

int main(){
    freopen("medie.in", "r", stdin);
    freopen("medie.out", "w", stdout);
    int i,n,j,k;
    long long unsigned ans = 0;
    ans = 0;
    scanf("%d" ,&n);
    for(i = 1;i <= n;i++){
        scanf("%d", &v[i]);
        frecv[2*v[i]]++;
    }
    for(j = 1;j <= n;j++){
        for(k = j+1;k <= n;k++){
            ans += frecv[v[j]+v[k]];
            if(v[j] == v[k]){
                ans -= 2;
            }
        }
    }
    printf("%lld",ans);
}