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