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