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