#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int i, j, k, l, n, m, p, t, a, b, f;
  cin >> n >> m >> k;
  l = (long long int)max(n, m);
  i = 0;
  f = 0;
  if (min(n, m) < 0 && l > 0 && k > max(n, m)) {
    p = -min(n, m);
    i = p / l;
    p -= i * l;
    n = -p;
    m = l;
  }
  while (k > l) {
    a = (long long int)min(n, m);
    b = (long long int)max(n, m);
    n = a + b;
    m = b;
    l = (long long int)max(n, m);
    i += 1;
    if (n <= a) {
      f = 1;
      break;
    }
  }
  if (f == 0)
    cout << i;
  else
    cout << "-1";
}
