#include <fstream>
#include <algorithm>

using namespace std;
long int mid,k,b,st,dr,numarul,i,j,n,v[3005];
int main()
{
    ifstream f("bete2.in");
    ofstream g("bete2.out");
    f>>n;
    for(i=1;i<=n;i++)
        f>>v[i];
    sort(v,v+n+1);
    for(i=1;i<n;i++)
        for(j=i+1;j<=n;j++)
        {
            b=0;
            numarul=v[i]+v[j];
            for(st=j,dr=n;st<=dr;)
            {
                mid=(st+dr)/2;
                if(v[mid]>numarul)
                    dr=mid-1;
                else if(v[mid]==numarul)
                {
                    k++; break;
                }
                else st=mid+1;
            }
        }
    g<<k;
}