#include <bits/stdc++.h>
using namespace std;

signed main(){
  double x1, y1, x2, y2; cin >> x1 >> y1 >> x2 >> y2;
  double xd = fabs( x1 - ( x1 + x2 ) / 2 );
  double yd = fabs( y1 - ( y1 + y2 ) / 2 );
  double ans = xd + yd;
  ans *= 2;
  int z = ( int ) ( ans + 1e-8 );
  if( z & 1 )
    cout << z / 2 << "." << 5 << endl;
  else
    cout << z / 2 << endl;
  return 0;
}