BOJ 31255 - ПОДНИЗОВЕ [C++]

문제

링크

풀이

#include <iostream>
#define MOD 123456789
using namespace std;
 
inline long long calc(long long x) {
  return (x*(x+1)/2) % MOD;
}
 
void solve(void) {
  long long n, p, q; cin >> n >> p >> q;
 
  cout << (calc(n) + MOD - calc(p-1) + MOD - calc(q-p-1) + MOD - calc(n-q)) % MOD;
}
 
int main(void) {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
 
  solve();
  return 0;
}