C++ :

#include<bits/stdc++.h>
using namespace std;
long long n,m,s;
int main(){
//	freopen("10.in","r",stdin);
//	freopen("10.out","w",stdout);
    cin>>m>>n;
    s=(1+n)*n/2;
    if(m>=s) cout<<0;
    else cout<<s-m;
    return 0;
}