#include <fstream>
#include <iomanip>
using namespace std;

ifstream fin("date.in");
ofstream fout("date.out");

float t1,k1,t2,k2,t;

int main()
{
    fin>>t1>>k1>>t2>>k2;
    t1/=k1;
    t2/=k2;
    t=t1+t2;
    t/=4;
    fout<<setprecision(2)<<t;

    fin.close();
    fout.close();
    return 0;
}