#include <bits/stdc++.h>
using namespace std;
long long x, y, k;
int main(void) {
  cin >> x >> y >> k;
  if (x <= 0 && y <= 0) {
    if (x >= k || y >= k)
      cout << 0 << endl;
    else
      cout << -1 << endl;
  } else {
    if (x > y) swap(x, y);
    long long cnt = 0;
    while (x < k && y < k) {
      if (x < 0) {
        long long cb = -x / y;
        cnt += cb;
        x += cb * y;
        while (x < 0) {
          cnt++;
          x += y;
        }
        continue;
      }
      x = x + y;
      if (x > y) swap(x, y);
      cnt++;
    }
    cout << cnt << endl;
  }
  return 0;
}
