HDU - 2114 Calculate S(n)

 1 #include <bits/stdc++.h>
 2 #define _xx ios_base::sync_with_stdio(0);cin.tie(0);
 3 #define INFS 0x3fffffff
 4 #define INFB 0x3fffffffffffffff
 5 #define pf printf
 6 #define sf scanf
 7 #define mem(a, b) memset(a, 0, sizeof a);
 8 using namespace std;
 9 typedef long long ll;
10 
11 int main()
12 {_xx
13     ll n;
14     while(cin >> n)
15     {
16         ll ans = n*(n + 1)/2;
17         ans %= 10000;
18         ans = ans*ans%10000;
19         cout << setw(4) << setfill(‘0‘) << ans << endl;
20     }
21     return 0;
22 }