#include<bits/stdc++.h>
using namespace std;
using ll=long long;
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
template<class T> bool chmax(T &a, T b){if (a < b){a = b;return true;} else return false;}
template<class T> bool chmin(T &a, T b){if (a > b){a = b;return true;} else return false;}

void Solve(){
    ll X,N;
    cin>>X>>N;
    bool flag=false;
    if(X<0){
        flag=true;
        X=-X;
    }
    ll ok=0,ng=sqrt(2*X)+10;
    while(ng-ok>1){
        ll mid=(ok+ng)/2;
        if(mid*(mid+1)/2<=X)ok=mid;
        else ng=mid;
    }
    ll ans;
    if(ok>=N){
        X-=N*(N+1)/2;
        ans=X;
    }else{
        X-=ok*(ok+1)/2;
        assert(X>=0);
        N-=ok;
        if(N%2==0)ans=X+N/2;
        else ans=X-(ok+1)-N/2;
    }
    if(flag)ans=-ans;
    cout<<ans<<"\n";
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    Solve();
}