保证AC

#include <bits/stdc++.h>
using namespace std;
int main() {
    int k;
    cin >> k;
    int coins = 0;
    int days = 0;
    int c = 1;
    while (days < k) {
        for (int i = 0; i < c && days < k; ++i) {
            coins += c;
            ++days;
        }
        ++ c;
    }
    cout << coins << endl;
}