#include <bits/stdc++.h>

#include "testlib.h"

void solve() {
    const long long m = 1'000'000'000'000'000'000;
    long long x       = inf.readLong(-m, m);
    inf.readSpace();
    long long n = inf.readLong(1, m);
    inf.readEoln();

    long long y = abs(x);
    long long l = 0;
    long long r = 1LL << 31;
    while (r - l > 1) {
        long long mid = (l + r) >> 1;
        if (mid * (mid + 1) / 2 > y) {
            r = mid;
        } else {
            l = mid;
        }
    }

    if (l > n) {
        if (x > 0) {
            x -= n * (n + 1) / 2;
        } else {
            x += n * (n + 1) / 2;
        }
        std::cout << x << std::endl;
        return;
    }

    y -= l * (l + 1) / 2;
    bool is_zero = y == 0;
    y += (n - l) / 2;
    if ((n - l) % 2 == 1) {
        y -= n;
    }
    if (is_zero or x <= 0) {
        std::cout << -y << std::endl;
    } else {
        std::cout << y << std::endl;
    }
}

int main(int argc, char *argv[]) {
    std::cin.tie(0)->sync_with_stdio(0);
    registerValidation(argc, argv);
    solve();
    inf.readEof();
}