num27.4

#include <iostream>
#include <fstream>
#include <vector>
using namespace std;

int main()
{
    ifstream input("C:\\Users\\Максим\\Downloads\\28130_A (1).txt");
    int n = 0, now = 0;
    vector<pair<int, int>> v(80); //pair: >50; <=50
    input >> n;
    for (size_t i = 0; i < n; i++)
    {
        input >> now;
        if (now > 50) {
            int k = now % 80;
            v[now % 80].first++;
        }
        else {
            v[now % 80].second++;
        }
    }
    int res = ((v[0].first - 1) * v[0].first) / 2;
    for (size_t i = 1; i < 40; i++)
    {
        res += v[i].first * v[80 - i].second + v[i].first * v[80 - i].first
            + v[i].second * v[80 - i].first;
    }
    res += v[40].first * v[40].second + ((v[40].first - 1) * v[40].first) / 2;
    cout << res;
}
/*https://inf-ege.sdamgia.ru/problem?id=28130*/