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