#include <bits/stdc++.h>
using namespace std;
int main() {
  long long x, y, m;
  cin >> x >> y >> m;
  if ((x >= m || y >= m)) {
    cout << "0\n";
    return 0;
  }
  if (x <= 0 && y <= 0) {
    cout << "-1\n";
    return 0;
  }
  long long res = 0;
  if (x <= 0) {
    long long p = abs(x) / y;
    res += p;
    x += p * y;
    if (x <= 0) {
      x += y;
      res++;
    }
  }
  if (y <= 0) {
    long long p = abs(y) / x;
    res += p;
    y += p * x;
    if (y <= 0) {
      y += x;
      res++;
    }
  }
  if (x > y) swap(x, y);
  if (x + y >= m) {
    ++res;
    cout << res;
    return 0;
  }
  long long f = 0, f1 = 1;
  for (int i = 0; i < 120; ++i) {
    if (f * x + f1 * y >= m) break;
    res++;
    f = f + f1;
    swap(f, f1);
  }
  cout << res;
  return 0;
}
