#include<stdio.h>

int a[9001],c[9001][2],w,n;
double b[9001];

void citire()
{
freopen("medie.in","r",stdin);
scanf("%d",&n);
for (int i=0; i<n; i++)
    scanf("%d",&a[i]);
fclose(stdin);     
}     

void medii()
{
for (int i=0; i<n; i++)     
    for (int j=i+1; j<n; j++)
        {
        b[++b[0]]=(double)((double)a[i]+(double)a[j])/2;
        c[b[0]][0]=i;
        c[b[0]][1]=j;
        }
}     

void medie()
{
w=0;
for (int i=1; i<=b[0]; i++)
    for (int j=0; j<n; j++)
        if (j!=c[i][1] && j!=c[i][0] && a[j]==b[i])
           ++w;
}     

void scriere()
{
freopen("medie.out","w",stdout);
printf("%d",w);
fclose(stdout);     
}     

int main()
{
citire();
medii();
medie();
scriere();
return 0;    
}