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