#include <bits/stdc++.h>
#define  Rep(i,j,n) for(li i=(j);i<(n);i++)
#define  rep(i,n) Rep(i,0,(n))
#define  all(a) begin(a),end(a)
using namespace std;
typedef long int li;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<string> vs;
 
int main()
{
    double a,b,c,d;
    scanf("%lf%lf%lf%lf",&a,&b,&c,&d);
    printf("%.1lf\n",(abs(a-c)+abs(b-d))/2.0);
    
    return 0;
}