#include <cstdio>
using namespace std;

#define Nmax 9002
#define Valmax 7002

FILE *f = fopen ( "medie.in", "r" );
FILE *g = fopen ( "medie.out", "w" );

short int v[Nmax], ap[Valmax];

int main(){

    int N, sol = 0;
    fscanf ( f, "%d", &N );

    for ( int i = 1; i <= N; ++i ){
        fscanf ( f, "%hd", &v[i] );
        ap[v[i]]++;
    }

    short int val;
    for ( int i = 1; i <= N; ++i ){
        for ( int j = i+1; j <= N; ++j ){
            val = (v[i]+v[j]) >> 1;
            if ( (v[i] + v[j]) & 1 )
                continue;
            sol += ap[val];
            sol -= ( val == v[i] );
            sol -= ( val == v[j] );
        }
    }

    fprintf ( g, "%d", sol );

    return 0;
}