#3335 caramele

#include <iostream>
using namespace std;
int main(){
    long long int c,p,x,r;
    cin >> c >> p;
    x = p * (p+1) / 2;
    r = c - x;
    r = r - r / p * p;
    if(r < 0)
        cout << c;
    else
        cout << r;
    return 0;
}