#include <fstream>
#include <algorithm>
using namespace std;

ifstream in("nrtri.in");
ofstream out("nrtri.out");

int sir[801], N;
bool marked[801];
int binary(int val)
{
    int step, i;
    for (step = 1; step < N; step <<= 1);
    for (i = 0; step; step >>= 1)
        if (i+step <= N && sir[i+step] <= val)
            i += step;
    return i;
}

int main()
{
    in >> N; int count = 0;
    for (int i = 1; i <= N; i++)
        in >> sir[i];
    sort(sir+1, sir+N+1);
    for (int i = 1; i < N; i++)
        for (int j = i+1; j <= N; j++)
        {
            int p = binary(sir[i] + sir[j]);
            if (p == N && sir[N] == sir[i]+sir[j] && !marked[N]) { count++; marked[N] = true; }
            else if (!marked[p]) { count++; marked[p] = true; }
        }

    out << count;
    return 0;
}