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