#include <bits/stdc++.h>
#pragma GCC optimize("O3")
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
using namespace std;
long long n, x, y, m, ans = 0;
int32_t main() {
  cin >> x;
  cin >> y;
  cin >> m;
  if (x >= m || y >= m) {
    cout << "0";
    return 0;
  }
  if (x <= 0 && y <= 0) {
    cout << "-1";
    return 0;
  }
  if (min(x, y) <= 0 && max(x, y) > 0) {
    if (x < y) {
      ans = -x / y;
      x = x % y;
    } else {
      ans = -y / x;
      y = y % x;
    }
  }
  while (x < m && y < m) {
    if (x < y) {
      x = x + y;
    } else {
      y = y + x;
    }
    ans++;
  }
  cout << ans;
  return 0;
}
