#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;
using ll = long long;
int main()
{
ll px,py; cin >> px >> py;
ll qx,qy;cin >> qx >> qy;
cout << fixed << setprecision(2)<< (abs(px-qx)+abs(py-qy))*1.0/2 << endl;
return 0;
}