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