#include <bits/stdc++.h>
using namespace std;
int main() {
  long long a, b, m;
  long long steps = 0ll;
  cin >> a >> b >> m;
  if (b > a) swap(a, b);
  if (a >= m)
    cout << 0 << endl;
  else if (a <= 0 && b <= 0)
    cout << -1 << endl;
  else {
    if (b < 0) {
      steps = -b / a + 1;
      b = b + a * steps;
    }
    while (a < m) {
      b = a + b;
      swap(a, b);
      steps++;
    }
    cout << steps << endl;
  }
  return 0;
}
