#include <bits/stdc++.h>
using namespace std;
 
#include <atcoder/all>
using namespace atcoder;
 
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
 
#define FASTIO cin.tie(0)->sync_with_stdio(0)
 
using ll = __int128;
using mint = modint998244353;

ll read() {
  string s;
  cin >> s;
  ll ret = 0;
  ll sgn = (isdigit(s[0]) ? 1 : -1);
  for(char &c : s) {
    if(!isdigit(c)) continue;
    ret = ret * 10 + (c - '0');
  }
  return sgn * ret;
}

void write(ll x) {
  if(x == 0) {
    cout << "0" << endl;
    return;
  }else if(x < 0) {
    cout << "-";
    x *= -1;
  }
  string ret = "";
  while(x > 0) {
    ret += to_string((int)(x % 10));
    x /= 10;
  }
  reverse(ret.begin(), ret.end());
  cout << ret << endl;
}

ll solve(ll x, ll n) {
  if(x == 0) {
    ll k = (n + 1) / 2;
    return (n % 2 == 1 ? k : -k);
  }else if(x < 0) {
    return -solve(-x, n);
  }else if(x > n * (n + 1) / 2) {
    return x - n * (n + 1) / 2;
  }
  ll ok = 0, ng = n;
  while(ok + 1 < ng) {
    ll k = (ok + ng) / 2;
    if(x > k * (k + 1) / 2) ok = k;
    else ng = k;
  }
  const ll m = ok;
  x -= m * (m + 1) / 2;
  if((n - m) % 2 == 1) {
    return x - (m + 1) - (n - m - 1) / 2;
  }else {
    return x + (n - m) / 2;
  }
}

int main() {
  ll x = read();
  ll n = read();
  write(solve(x, n));
}