#include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <math.h>
#include <cmath>
#include <limits.h>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <functional>
#include <stdio.h>
using namespace std;

long long MOD = 1000000007;

int main() {
    
    double PX,PY,QX,QY;
    cin >> PX >> PY >> QX >> QY;
    
    printf( "%.1f", ( abs( PX - QX ) ) / 2.0 + ( abs( PY - QY ) ) / 2.0 );
    
    
    return 0;
}