#include <bits/stdc++.h>
using namespace std;
const int maxn = 100 * 1000 + 10, maxm = (1 << 17), mod = (int)1e9 + 7,
          hash = 701, inf = (1 << 29) + 3;
const double pi = 3.14159265359, ee = 2.71828;
int main() {
  ios::sync_with_stdio(0);
  long long x, y, m, ans = 0;
  cin >> x >> y >> m;
  if (x < y) swap(x, y);
  if (x >= m) {
    cout << 0 << endl;
    return 0;
  }
  if (x < 1) {
    cout << -1 << endl;
    return 0;
  }
  ans = (2 * x - y - 1) / x, y += x * ans;
  while (1) {
    swap(x, y);
    if (x >= m) {
      cout << ans << endl;
      return 0;
    }
    ans++, y += x;
  }
  return 0;
}
