#include <bits/stdc++.h>

using namespace std;


typedef int32_t h_int32;
typedef int64_t h_int64;
typedef uint32_t uh_int32;
typedef uint64_t uh_int64;
typedef h_int64 h_int;

#define REP( i, n ) for(int i = 0; i < ( int )( n ); ++i )
#define REPR( i, n ) for(int i = ( int )( n ); i >= 0; --i)
#define FOR( i, a, n ) for(int i = ( int )( a ); i < ( int )( n ); ++i )
#define FORR( i, a, n ) for(int i = ( int )( n ); i >= ( int )( a ); --i)

#define VINIT( v, type, n )vector<type> v; v.resize( n );
#define VINIT2( v, type, n, a )vector<type> v; v.resize( n, a );
#define VCIN( v, type, n ) VINIT( v, type, n ) REP( i, n ) cin >> v[i];
#define ALL( x ) x.begin(), x.end()

#define DOUT( x ) cerr << #x << " = " << x << "\n";
#define COUT( x ) cout << ( x ) << "\n";

#define INF 999999999



signed main()
{
        ios::sync_with_stdio( false );
        cin.tie( 0 );

        int a, b, x, y;
        cin >> a >> b;
        cin >> x >> y;
        int dx = abs( a - x );
        int dy = abs( b - y );
        COUT( ( double )( dx + dy ) / 2.0 );
        return 0;
}