Finding inverse

#include <iostream>
#include <numeric>
using namespace std;
#define int long long


int euler(int n) {
    int c = 0;
    for (int i = 1; i <= n; ++i) {
        if (gcd(n, i) == 1) {
            c++;
        }
    }
    return c;
}


int fast_exp(int a, int b, int mod) {
    if (b == 0) {
        return 1;
    }
    int g = 1;
    while (b > 1) {
        if (b % 2 == 1) {
            g = (g * a) % mod;
            b--;
        }
        a = (a * a) % mod;
        b /= 2;
    }
    return (a * g) % mod;
}

signed main() {
    long long a, M;
    cin >> a >> M;

    if (gcd(a, M) != 1) {
        cout << -1 << endl;
    } else {
        int e = euler(M);
        cout << fast_exp(a, e - 1, M) << endl;
    }

    return 0;
}

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: ACCEPTED