#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 << endl;
    return 0;
  }
  if (a <= 0 && b <= 0) {
    cout << -1 << endl;
    return 0;
  }
  long long ans = 0;
  if (b < 0) {
    ans += (-b) / a + 1;
    b %= a;
    b += a;
  }
  while (a < m) {
    long long c = a;
    a = a + b;
    b = c;
    ans++;
  }
  cout << ans << endl;
  return 0;
}
