#include <iostream>
#include <set>
#include <string>
using namespace std;

typedef unsigned long long ull;

int main() {
    ull N, M;
    int ans = 0, times = 0;
    set<ull> recorder;
    cin >> N >> M;

    while (N != 0) {
        times++;
        if (times > 200) {
            cout << -1 << endl;
            return 0;
        }
        /*
        if (recorder.find(N) != recorder.end()) {
            cout << -1 << endl;
            return 0;
        }
        recorder.insert(N);
        */
        string str = to_string(N / M);
        for (auto ch : str) {
            if (ch != '0') ans = ch - '0';
        }
        N = (N % M) * 10;
    }
    cout << ans << endl;
    return 0;
}