#include <bits/stdc++.h>
using namespace std;
long long a, b, m, cnt;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cin >> a >> b >> m;
  if (a >= m || b >= m) {
    cout << 0 << "\n";
    return 0;
  }
  if (a <= 0 && b <= 0) {
    cout << -1 << "\n";
    return 0;
  }
  if (a > b) swap(a, b);
  if (a < 0) {
    cnt = abs(a) / b;
    a += cnt * b;
  }
  while (a < m && b < m) {
    if (a < b)
      a += b;
    else
      b += a;
    ++cnt;
  }
  cout << cnt << "\n";
  return 0;
}
