[C++백준 4641번:Doubles

#include <iostream>
#include <vector>
using namespace std;
 
int main() {
    int n;
    while (true) {
        vector<int> v;
        for (int i = 0; i < 20; i++) {
            cin >> n;
            if (n == -1) {
                return 0;
            }
            if (n == 0) {
                break;
            }
            v.push_back(n);
        }
 
        int count = 0;
        for (int i = 0; i < v.size(); i++) {
            for (int j = 0; j < v.size(); j++) {
                if (v[i] * 2 == v[j]) {
                    count++;
                }
            }
        }
        cout << count << '\n';
    }
}