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