#include <bits/stdc++.h>
using namespace std;
int main() {
  long long x, y, m, i = 0;
  cin >> x >> y >> m;
  if (m <= x || m <= y) {
    cout << "0\n";
    return 0;
  }
  if (x <= 0 && y <= 0 && m > x && m > y) {
    cout << "-1\n";
    return 0;
  }
  if (m < 0) {
    m *= -1;
    x *= -1;
    y *= -1;
  }
  if (x > y) swap(x, y);
  if (y < m && x < 0 && y > 0) {
    long long a = -1 * x, b = y;
    i += a / b;
    x = -(a % b);
  }
  while (true) {
    if (x >= m || y >= m) {
      cout << i << endl;
      return 0;
    }
    i++;
    if (x > y) swap(x, y);
    x = x + y;
  }
  return 0;
}
