#include <cstdio>
#include <algorithm>

using namespace std;

#define inFile "medie.in"
#define outFile "medie.out"
#define MAX_N 9000
#define MAX_VAL 7000

int v[MAX_N+1];
int freq[MAX_VAL+1];
int nr[MAX_VAL*2+1];

int main() {
    freopen(inFile, "r", stdin);
    freopen(outFile, "w", stdout);
    
    int n, i, j;
    long long s = 0;
    
    scanf("%d", &n);
    
    for(i = 1; i <= n; i++) {
        scanf("%d", &v[i]);
        freq[v[i]]++;
    }
    
    for(i = 1; i <= n; i++)
        for(j = i+1; j <= n; j++)
            nr[v[i] + v[j]]++;
    
    for(i = 1; i <= n; i++) {
        if(nr[2*v[i]] > 0)
            s += nr[2*v[i]] - freq[v[i]] + 1;
    }
    
    printf("%lld\n", s);
    
    return 0;
}