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