C++ :

#include <iostream>
using namespace std;

int main(){
    int n, coin = 1,day=0, sum=0;
    cin >> n;
    for(int i = 1; i <= n; ++i){
        if(day >= n) break;
        for(int j = 1; j <= i; ++j){
            sum += coin;
            day++;
            if(day >= n) break;
        }
        coin++;
    }
    cout << sum << endl;
    return 0;
}