#include <stdio.h>

#define Nmax 7001

int v[Nmax],n,nr,max;

int main()
{
    freopen("medie.in","r",stdin);
    freopen("medie.out","w",stdout);
    int i,x;
    scanf("%d",&n);
    for(i=1;i<=n;++i)
        {
            scanf("%d",&x);
            ++v[x];
            if(x>max)
                max=x;
        }
    for(i=1;i<=max;++i)
        {
            if(v[i]>2)
                nr+=v[i]*(v[i]-1)*(v[i]-2)/2;
            for(int j=1;j<=max;++j)
                if(i+j<=max)
                  nr+=v[i-j]*v[i+j];
        }
    printf("%d",nr);       
    return 0;
}