poj 1552 Doubles

#include <iostream>using namespace std;int num[20];int main(){    int n, i, j, c;    while (cin >> num[0] && num[0] != -1)    {          n = c = 0;          while (num[n])          {                n++;                cin >> num[n];          }          for (i = 0; i < n; i++)             for (j = i+1; j < n; j++)             {                 if (num[i] == num[j] * 2 || num[i] * 2 == num[j])                    c++;             }          cout << c << endl;    }        system("pause");}