#include <bits/stdc++.h>
using namespace std;
int main() {
  long long a, b, m;
  cin >> a >> b >> m;
  long long ans;
  if (m <= max(a, b)) {
    cout << 0;
    return 0;
  }
  if (a + b <= min(a, b)) {
    cout << -1;
    return 0;
  }
  if (a < 0) {
    long long k = abs(a) / b;
    a += k * b;
    ans = k;
  }
  if (b < 0) {
    long long k = abs(b) / a;
    b += k * a;
    ans = k;
  }
  for (; max(a, b) < m; ans++)
    if (a < b)
      a += b;
    else
      b += a;
  cout << ans;
  return 0;
}
