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