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