#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long int x, y, m;
  cin >> x >> y >> m;
  if (x <= 0 && y <= 0 && x < m && y < m) {
    cout << -1 << "\n";
    return;
  }
  if (x >= m || y >= m) {
    cout << 0 << "\n";
    return;
  }
  long long int ans = 0;
  long long int mina = min(x, y);
  long long int maxa = max(x, y);
  if (mina < 0) {
    long long int t = ceil((long double)abs(mina) / (long double)maxa);
    ans += t;
    mina += t * maxa;
  }
  if (mina < m && maxa < m) {
    while (maxa < m) {
      ans++;
      mina += maxa;
      swap(mina, maxa);
    }
  }
  cout << ans << "\n";
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int t, q;
  q = 1;
  for (t = 1; t <= q; t++) {
    solve();
  }
}
