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