#include <bits/stdc++.h>
using namespace std;
long long a, b, m, ans;
int main() {
  cin >> a >> b >> m;
  if (a >= m || b >= m) {
    cout << 0;
    return 0;
  }
  if (a < 1 && b < 1) {
    if (a >= m || b >= m)
      cout << 0;
    else
      cout << -1;
  } else {
    if ((a < b ? a : b) < 0) {
      if (b < a) swap(a, b);
      long long k = (-1 * a) + b, p;
      p = k / b;
      ans = p;
      a += b * p;
    }
    while (max(a, b) < m) {
      if (a < b)
        a += b;
      else
        b += a;
      ans++;
    }
    cout << ans;
  }
}
