#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int n, m, t, s = 0;
  scanf("%I64d%I64d%I64d", &n, &m, &t);
  if (n > m) swap(n, m);
  if (n >= t || m >= t) {
    cout << "0" << endl;
  } else if (m <= 0) {
    cout << "-1" << endl;
  } else {
    if (n < 0) {
      s = -n / m;
      n += s * m;
    }
    while (n < t && m < t) {
      ++s;
      n = m + n;
      if (n > m) swap(n, m);
    }
    cout << s << endl;
  }
}
