#include <bits/stdc++.h>
using namespace std;
template <typename T>
string toString(T x) {
  if (x == 0) return "0";
  bool negative = x < 0;
  string res;
  while (x) {
    res.push_back('0' + x % 10);
    x /= 10;
  }
  if (negative) res.push_back('-');
  reverse(res.begin(), res.end());
  return res;
}
void ioFromFile(string inputFileName, string outputFileName) {
  ifstream fin(inputFileName);
  ofstream fout(outputFileName);
  cin.rdbuf(fin.rdbuf());
  cout.rdbuf(fout.rdbuf());
}
int read() {
  int res;
  scanf("%d", &res);
  return res;
}
void write(int x) { printf("%d ", x); }
template <typename T>
T gcd(T a, T b) {
  return a == 0 ? b : gcd(b % a, a);
}
template <typename T>
T lcm(T a, T b) {
  return a / gcd(a, b) * b;
}
int main() {
  long long a, b, k;
  cin >> a >> b >> k;
  if (a > b) swap(a, b);
  if (b >= k) {
    cout << 0;
    return 0;
  }
  if (b <= 0) {
    cout << -1;
    return 0;
  }
  long long res = 0;
  if (a < 0) {
    res = (-a) / b + 1;
    a += res * b;
  }
  while (b < k) {
    res++;
    a += b;
    swap(a, b);
  }
  cout << res;
  return 0;
}
