#include <bits/stdc++.h>
using namespace std;
inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T>
inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}
const double EPS = 1e-10;
const double PI = acos(-1.0);
int main() {
  long long x, y, m;
  cin >> x >> y >> m;
  if (x > y) swap(x, y);
  if (y < m && x + y <= x) {
    cout << -1 << endl;
    return 0;
  }
  long long cnt = 0;
  if (x < 0 && y < m) {
    cnt += abs(x) / y;
    x += y * cnt;
  }
  if (x > y) swap(x, y);
  while (y < m) {
    long long t = y;
    y = y + x;
    x = t;
    if (x > y) swap(x, y);
    cnt++;
  }
  cout << cnt << endl;
}
