
#include 
#include 
#include 
#include 


using std::cin;
using std::cout;
using std::endl;
using std::vector;
using std::pair;


int main()
{
	vector> n;

	for (int i = 0; i < 4; i++)
	{
		float tmp;
		cin >> tmp;

		n.push_back({i + 1, tmp});
	}

	
	int res = -1;
	for (int i = 1; i <= n.size(); i++)
	{
		int i0 = (i - 1) % n.size(), 
		    i1 =  i % n.size(), 
		    i2 = (i + 1) % n.size();
		if (n[i0].second /(float)n[i0].first != n[i1].second / (float)n[i1].first &&
			n[i1].second /(float)n[i1].first != n[i2].second / (float)n[i2].first)
			res = i1;		
	}
	
	cout << n[res].second << endl;
	return 0;
}