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