#include <iostream>
#include <cmath>
#include <fstream>
#define DMAX 805
using namespace std;

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

int v[DMAX], nr;
bool pus[DMAX];

int main()
{
    int n, i, j;
    fin >> n;
    for(i = 1; i <= n; i++)
        fin >> v[i];

    bool sortat;
    do
    {
        sortat = 0;
        for(i = 1; i < n; i++)
            if(v[i] > v[i+1])
                {
                    int aux = v[i];
                    v[i] = v[i+1];
                    v[i+1] = aux;
                    sortat = 1;
                }
    }
    while(sortat);

    int val;
    for(i = 1; i < n; i++)
        for(j = i + 1 ; j <= n; j++)
            if(!pus[i] || !pus[j])
            {
               val = v[i] + v[j];
               pus[i] = 1;
               pus[j] = 1;

               int st = 0, dr = n + 1;
               while(dr - st > 1)
               {
                   int mij = (st + dr)/2;

                   if(v[mij] >= val)
                    dr = mij;
                    else
                        st = mij;
               }
               if(dr <= n && v[dr] == val)
               {
                   nr ++;
                   pus[dr] = 1;
               }
               else if(v[st] < val)
                {
                    nr ++;
                    pus[st] = 1;
                }

            }
    fout << nr;
    return 0;
}