Finding inverse

#include <iostream>
#include <set>

using namespace std;

typedef long long ll;



int main(){
    ll a,b;
    cin >> a >> b;
    set<ll> s;
    bool keep=true;
    int idx=1;
    while(keep){
        if((idx*a)%b==1)break;
        else if(s.find((idx*a)%b)!=s.end()){
            keep=false;
        }
        s.insert((idx*a)%b);
        idx+=1;
    }
    if(keep) cout << idx << endl;
    else cout << -1 << endl;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: TIME LIMIT EXCEEDED

Test 13

Verdict: TIME LIMIT EXCEEDED

Test 14

Verdict: TIME LIMIT EXCEEDED