#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define abs(x) ( (x) < 0 ? -(x) : (x) )

int N,h[7001],v[9000];

int main() {

        freopen("medie.in","r",stdin);
        freopen("medie.out","w",stdout);

        scanf("%d",&N);

        for(int i=0;i<N;++i) {
                scanf("%d",&v[i]);
                h[ v[i] ] ++;
        }

        int res = 0;
        for(int i=0;i<N-1;++i)
         for(int j=i+1;j<N;++j)
           if(v[i] == v[j]) res += h [ v[i] ] - 2;
            else res += abs(v[i] - v[j]) & 1 ? 0 : h[ (v[i] + v[j]) / 2 ];

        printf("%d",res);
}