#include<iostream>
#include<cmath>
using namespace std;
void ans(unsigned long long n);
int main(){
    unsigned long long n;
    while(cin >> n){
        ans(n);
    }
}
void ans(unsigned long long n){
    int a=1,b=1,c=-2*n;
    cout << (long long)((-b+sqrt(b*b-4*a*c))/2*a) << "\n";
    //cout << (1+n)*n/2 << "\n";
}

為什麼這樣不行