#include <stdio.h>
#include <stdlib.h>
void caut_bin(int,int,int);
void qs(int,int);
void sw(int *a,int *b);
int a[801],m,n;

void sw(int *a,int *b)
{
    int t=*a; *a=*b; *b=t;
}

void qs(int left,int right)
{
    int i,j,p;
    i=left; j=right; p=a[(i+j)/2];
    while (i<j)
    {
        while (a[i]<p) i++;
        while (a[j]>p) j--;
        if (i<=j) {sw(&a[i],&a[j]); i++; j--;}
    }
    if (i<right) qs(i,right);
    if (j>left) qs(left,j);
}

void caut_bin(int x,int i,int j)
{
    int r=(i+j)/2;
    if (i==j && a[i]<=x) m++;
    if (i!=j)
    if (x<=a[r]) caut_bin(x,i,r); else
    {
        caut_bin(x,i,r);
        caut_bin(x,r+1,j);
    }
}

int main()
{
    int i,j;
    freopen("nrtri.in","r",stdin);
    freopen("nrtri.out","w",stdout);
    scanf("%i",&n);
    for (i=1; i<=n; i++) scanf("%i",&a[i]);
    qs(1,n);
    m=0;
    for (i=1; i<=n-2; i++)
    {
     for (j=i+1; j<=n-1; j++)
      {
        caut_bin(a[i]+a[j],j+1,n);
      }
    }
    printf("%i",m);
    return 0;
}