#include <fstream>
#include <algorithm>
using namespace std;
ifstream in("nrtri.in");
ofstream out("nrtri.out");
int main(){
int i, n, st, dr, gasit=0, m, j, S, nrtr=0;
in>>n;
int v[n+1];
for(i=1; i<=n; i++)
    in>>v[i];
sort(v+1, v+n+1);
    for(i=1; i<n-1; i++){
        for(j=i+1; i<n; i++){
            gasit=0;
            S=v[i]+v[j];
            st=j+1; dr=n;
            while(st<=dr&&!gasit)
            {
                m=(st+dr)/2;
            if(S==v[m])gasit=1;
            else st=m+1;
            }

        }
    }nrtr=nrtr+m-j;
out<<nrtr;
return 0;
}