OI-codes/AcWing/203/203.cpp

#include <iostream>

using std::cin;
using std::cout;
const char endl = '\n';

int a, b, x, y;

int exgcd(int a, int b, int &x, int &y) {
    if (!b) {
        x = 1;
        y = 0;
        return a;
    }

    int g = exgcd(b, a % b, y, x);
    y -= a / b * x;

    return g;
}

int main() {
    std::ios::sync_with_stdio(false);

    cin >> a >> b;

    exgcd(a, b, x, y);

    cout << (x % b + b) % b << endl;

    return 0;
}