#include <cstdio>
#include <cstring>

using namespace std;

#define NMAX 9001

long long sol;
int A[NMAX],mainsel[NMAX];
int i,N,j,medie;

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

scanf("%d",&N);

for (i=1;i<=N;++i)
{
    scanf("%d",&A[i]);
    ++mainsel[A[i]];
}

for (i=1;i<N;++i)
for (j=i+1;j<=N;++j)
{
    if (A[i]+A[j]&1==1) continue;

    medie=A[i]+A[j]>>1;

    if (medie>7000) continue;

    if (medie==A[i])
    --sol;
    if (medie==A[j])
    --sol;

    sol+=mainsel[medie];
}

printf("%lld",sol);

return 0;
}