#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int x, y, m;
  cin >> x >> y >> m;
  long long int ans = 0;
  bool f = 0;
  if (x > y) {
    long long int temp = x;
    x = y;
    y = temp;
  }
  if (x < 0 && y > 0 && y < m) {
    ans += (x * -1 + y - 1) / y;
    x = ans * y + x;
  }
  while (1) {
    if (x >= m || y >= m) break;
    if (x > y) {
      long long int temp = x;
      x = y;
      y = temp;
    }
    long long int temp = x + y;
    if (temp <= x) {
      f = 1;
      break;
    }
    x = temp;
    ans++;
  }
  if (f)
    cout << -1 << endl;
  else
    cout << ans << endl;
  return 0;
}
