#include <bits/stdc++.h>
using namespace std;
int main() {
  long long x, y, m, ans = 0;
  cin >> x >> y >> m;
  if (x >= m || y >= m) return cout << 0 << endl, 0;
  if (x <= 0 && y <= 0) return cout << -1 << endl, 0;
  if (x > y) swap(x, y);
  if (x < 0) {
    ans += (-x + y - 1) / y;
    x = x + ans * y;
  }
  if (x > y) swap(x, y);
  while (y < m) {
    x = x + y;
    if (x > y) swap(x, y);
    ++ans;
  }
  cout << ans << endl;
  return 0;
}
