#include <atcoder/all>
#include <bits/stdc++.h>
#define rep(i, a, b) for (ll i = (ll)(a); i < (ll)(b); i++)
using namespace atcoder;
using namespace std;

typedef long long ll;

int main() {
    cin.tie(0);
    cout.tie(0);
    ios::sync_with_stdio(0);
    ll x, n;
    cin >> x >> n;
    auto check = [&](ll mid) { // ([...,1,1,1,0,0,0,...])
        if (mid * (mid + 1) / 2 >= x)
            return true;
        return false;
    };
    auto binary = [&]() {
        ll ac = 1414213572, wa = 0;
        while (ac - wa > 1) {
            ll mid = (ac + wa) / 2;
            if (check(mid))
                ac = mid;
            else
                wa = mid;
        }
        return ac;
    };
    ll minus_ind = binary();
    if (n <= minus_ind) {
        cout << x - n * (n + 1) / 2 << endl;
        return 0;
    }
    ll mi = x - minus_ind * (minus_ind + 1) / 2;
    ll pl = mi + minus_ind + 1;
    ll amari = n - minus_ind;
    ll ans;
    if (amari & 1LL) {
        ans = pl + amari / 2;
    } else {
        ans = mi - amari / 2;
    }
    cout << ans << endl;
}