#include <bits/stdc++.h>
using namespace std;
int main()
{
    double PX, PY, QX, QY;
    cin >> PX >> PY >> QX >> QY;
    cout << fixed << setprecision( 1 ) << ( abs( PX - QX ) + abs( PY - QY ) ) / 2.0 << endl;
    return 0;
}