#include <bits/stdc++.h>
using namespace std;
long long n, m, x, y, l, r, ql, qr, ans, s, a[300100], dpl[300100], dpr[300100];
int main() {
  cin >> x >> y >> n;
  if (x > y) swap(x, y);
  if (y >= n) return cout << 0, 0;
  if (x <= 0 && y <= 0 && n > y) return cout << -1, 0;
  if (x < 0 && y > 0) {
    ans += ((-x) / y);
    a[0] = x;
    a[1] = x - (x / y) * y;
    a[2] = y;
    sort(a, a + 3);
    x = a[1];
    y = a[2];
  }
  while (y < n) {
    ans++;
    s = x + y;
    a[0] = x;
    a[1] = y;
    a[2] = s;
    sort(a, a + 3);
    x = a[1];
    y = a[2];
  }
  cout << ans;
  return 0;
}
