同余方程

#include <bits/stdc++.h>
using namespace std;

#define int long long

int exgcd(int a,int b, int &x, int &y)
{
    if(a<b) return exgcd(b,a,y,x);
    if(b==0){
        x = 1; y = 0;
        return a;
    }
    else{
        int x1;
        int d = exgcd(b,a%b,x1,x);
        y = x1 - a/b * x;
        return d;
    }
}

signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);cout.tie(0);

    int t;cin >> t;
    while(t--)
    {
        int a,b;cin >> a >> b;
        int x,y;
        int m = exgcd(a,b,x,y);
        if(m!=1) cout << -1 << endl;
        else cout << (x % b + b) % b << endl;
    }

    return 0;
}