#include <bits/stdc++.h>
#pragma GCC optimize("unroll-loops")
#pragma GCC optimize("Ofast")
#pragma GCC optimize("no-stack-protector")
#pragma GCC target("sse,sse2,sse3,ssse3,popcnt,abm,mmx,tune=native")
#pragma GCC optimize("fast-math")
#pragma GCC optimize "-O3"
using namespace std;
const long long INF = 1e9;
const long long INFLL = 1e12;
const long long MOD = 1000000007;
void fast_io() {}
signed main() {
  fast_io();
  long long a, b, n;
  cin >> a >> b >> n;
  a %= b;
  a *= 10;
  for (long long i = 1; i <= b; i++) {
    if (a / b == n) {
      cout << i;
      return 0;
    }
    a %= b;
    a *= 10;
  }
  cout << -1;
  return 0;
}
