#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vi = vector<int>;
using pi = pair<int, int>;
ll x, y, m;
void solve() {
  cin >> x >> y >> m;
  if (x <= 0 && y <= 0 && m > x && m > y) {
    cout << "-1\n";
    return;
  }
  if (x >= m || y >= m) {
    cout << "0\n";
    return;
  }
  ll ans = 0;
  if (x > y) swap(x, y);
  if (x < 0) {
    ans = -x / y;
    x += y * ans;
  }
  while (x < m && y < m) {
    if (x > y) swap(x, y);
    if (y + x >= m) {
      y += x;
      ++ans;
    } else {
      x += y;
      ++ans;
    }
  }
  cout << ans << '\n';
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int t;
  t = 1;
  while (t--) solve();
}
