#include <bits/stdc++.h>
using namespace std;
long long a, b, m;
int main() {
  cin >> a >> b >> m;
  if (a < b) swap(a, b);
  if (a <= 0 && b <= 0) {
    if (a >= m)
      printf("0\n");
    else
      printf("-1\n");
    return 0;
  }
  long long ans = 0;
  if (b <= 0) {
    if (a >= m) {
      printf("0\n");
      return 0;
    }
    ans = -b / a;
    b += a * ans;
    if (b <= 0) {
      b += a;
      ans++;
    }
  }
  if (a < b) swap(a, b);
  long long temp;
  while (a < m) {
    temp = a;
    a += b;
    b = temp;
    ans++;
  }
  cout << ans << endl;
  return 0;
}
