#include <bits/stdc++.h>
using namespace std;
const int N = 500010;
long long x, y, m;
int main() {
  ios_base::sync_with_stdio(false);
  cin >> x >> y >> m;
  if (x >= m || y >= m) {
    cout << "0\n";
    return 0;
  }
  if (x <= 0 && y <= 0) {
    cout << "-1\n";
    return 0;
  }
  if (x > y) swap(x, y);
  long long res = 0;
  if (x < 0) {
    res = ceil(-x * 1.0 / y);
    x += res * y;
  }
  while (x < m && y < m) {
    if (x < y)
      x = x + y;
    else
      y = x + y;
    res++;
  }
  cout << res << "\n";
  return 0;
}
