#include <iostream>
#include <list>
#include <vector>
#include <string>
#include <stdio.h>
#include <stdint.h>
using namespace std;

using uint = unsigned int;
using ll = long long;

#define CIN( LL , A ) LL A; cin >> A 
#define GETLINE( A ) string A; getline( cin , A ) 
#define FOR_ITR( ARRAY , ITR , END ) for( auto ITR = ARRAY .begin() , END = ARRAY .end() ; ITR != END ; ITR ++ ) 

#include <iomanip>

int main()
{

  CIN( ll , PX );
  CIN( ll , PY );
  CIN( ll , QX );
  CIN( ll , QY );

  // min + ( max - min ) / 2 = ( max + min ) / 2 = sum / 2;
  double answer = ( ( PX < QX ? QX - PX : PX - QX ) + ( PY < QY ? QY - PY : PY - QY ) ) / 2.0;
  cout << fixed << setprecision( 1 ) << answer << endl;
  return 0;

}