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