#include <bits/stdc++.h>
int main() {
  long long int x, y, m;
  std::cin >> x >> y >> m;
  if (x >= m || y >= m) {
    std::cout << "0\n";
    return 0;
  }
  if (x <= 0 && y <= 0) {
    std::cout << "-1\n";
    return 0;
  }
  long long int count = 0;
  while (x < m && y < m) {
    if (x < y) {
      long long int t = (y - x) / y;
      if (t <= 0) t = 1;
      x += t * y;
      count += t;
    } else {
      long long int t = (x - y) / x;
      if (t <= 0) t = 1;
      y += t * x;
      count += t;
    }
  }
  std::cout << count << "\n";
  return 0;
}
