2021 HSG9 DANANG B3. DTLN Diện tích lớn nhất

#include <bits/stdc++.h>
using namespace std;

int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);

    freopen("dientich.inp","r",stdin);
    freopen("dientich.out","w",stdout);

    long long ans = 0, u;
    cin >> k;

    for (long long i = 1; i < k; i++){
        double v = sqrt(k * k - i * i);
        if (int(v) == v) ans = max(ans, i * int(v)); 
    }

    cout << ans;
}