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