#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, c, op = 0, q;
  cin >> a >> b >> c;
  do {
    a *= 10;
    q = a / b;
    op++;
    a = a % b;
  } while ((op <= b) && (q != c));
  if (op <= b)
    cout << op;
  else
    cout << -1;
  return 0;
}
