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