#include <bits/stdc++.h>
using namespace std;
int main() {
  long long smol, chungus, h, res = 0;
  cin >> smol >> chungus >> h;
  if (max(smol, chungus) >= h) {
    cout << 0 << endl;
    return 0;
  }
  if (smol <= 0 && chungus <= 0) {
    cout << -1 << endl;
    return 0;
  }
  while (smol < h && chungus < h) {
    if (smol > chungus) swap(smol, chungus);
    if (smol < 0) {
      res += abs(smol) / chungus;
      smol += res * chungus;
    }
    ++res;
    smol += chungus;
  }
  cout << res;
  return 0;
}
