#include<stdio.h>

int v[7002];

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