#include <bits/stdc++.h>

using namespace std;

void fast_io() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
}
std::ostream &operator<<(std::ostream &dest, __int128_t value) {
    std::ostream::sentry s(dest);
    if (s) {
        __uint128_t tmp = value < 0 ? -value : value;
        char buffer[128];
        char *d = std::end(buffer);
        do {
            --d;
            *d = "0123456789"[tmp % 10];
            tmp /= 10;
        } while (tmp != 0);
        if (value < 0) {
            --d;
            *d = '-';
        }
        int len = std::end(buffer) - d;
        if (dest.rdbuf()->sputn(d, len) != len) {
            dest.setstate(std::ios_base::badbit);
        }
    }
    return dest;
}

int main() {
    fast_io();
    __int128_t x, n;
    {
        long long xx, nn;
        cin >> xx >> nn;
        x = xx;
        n = nn;
    }
    bool swapped = x <= 0;
    if (swapped) x = -x;
    if (x >= n * (n + 1) / 2) {
        __int128_t ans = x - n * (n + 1) / 2;
        if (swapped) {
            ans = -ans;
        }
        cout << ans << endl;
        return 0;
    }
    __int128_t ok = 0, ng = n;
    while (ng - ok > 1) {
        __int128_t mid = (ok + ng) / 2;
        if (mid * (mid + 1) / 2 >= x) {
            ng = mid;
        } else {
            ok = mid;
        }
    }
    __int128_t ans = x - ok * (ok + 1) / 2;
    __int128_t res_op = n - ok;
    ans += res_op / 2;
    if (res_op % 2 == 1) {
        ans -= n;
    }
    if (swapped) {
        ans = -ans;
    }
    cout << ans << endl;
}