#include <bits/stdc++.h>
using namespace std;
const int MAXN = 10 + 1e6;
const long long oo = 1 + 1e18, mod = 7 + 1e9;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0), cout.tie(0);
  long long x, y, m;
  cin >> x >> y >> m;
  if (x >= m || y >= m) {
    cout << 0;
    return 0;
  }
  if (x <= 0 && y <= 0) {
    cout << -1;
    return 0;
  }
  long long ans = 0;
  while (x < m && y < m) {
    if (x > y) swap(x, y);
    long long need = (y - x) / y + 1;
    ans += need;
    x += y * need;
  }
  if (x >= m || y >= m)
    cout << ans;
  else
    cout << -1;
  return 0;
}
