#include <bits/stdc++.h>
using namespace std;
long long maxm(long long x, long long y) { return (x > y) ? x : y; }
int main() {
  long long x, y, m, cnt = 0;
  cin >> x >> y >> m;
  if (x >= m || y >= m) {
    cout << "0\n";
  } else if (x <= 0 && y <= 0)
    cout << "-1\n";
  else {
    while (true) {
      if (x > y) swap(x, y);
      cnt += (maxm(y - x, y)) / y;
      x += (maxm(y - x, y) / y) * y;
      if (x >= m || y >= m) {
        cout << cnt << "\n";
        return 0;
      }
    }
  }
  return 0;
}
