http://www.spoj.com/problems/BALLSUM/

main.cc

#include <bits/stdc++.h>
 
using namespace std;
 
using ll = long long;
 
ll gcd(ll a, ll b) {
    return !b ? a : gcd(b, a % b);
}
 
int main(int argc, char **argv) {
    ios::sync_with_stdio(false);
    srand(time(0));
    while (true) {
        ll n, k;
        cin >> n >> k;
        if (n == -1 && k == -1) break;
        ll a = 0;
        for (ll i = 1; 2*i <= k; ++i) a += k - 2*i;
        ll b = n*(n-1)/2;
        ll ggcd = gcd(a, b);
        if (a == 0) cout << 0 << endl;
        else if (a == b) cout << 1 << endl;
        else cout << a/ggcd << "/" << b/ggcd << endl;
    }
    return 0;
}