#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int x, y, n, counter = 0;
  cin >> x >> y >> n;
  if (x >= n || y >= n) {
    cout << "0";
    return 0;
  }
  if (x + y <= x && x + y <= y) {
    cout << "-1";
    return 0;
  }
  if (x < 0 && y > 0) {
    x = -x;
    counter += x / y;
    x = -x % y;
  }
  if (y < 0 && x > 0) {
    y = -y;
    counter += y / x;
    y = -y % x;
  }
  while (1) {
    if (x >= n || y >= n) break;
    if (x <= y)
      x = x + y;
    else
      y = x + y;
    counter++;
  }
  cout << counter;
}
