#include<stdio.h>
int main(){
    double abs[4],X,Y;
    int i;
    
    for(i=0;i<4;i++){
        scanf("%lf",&abs[i]);
    }
    
    if(abs[1]<0 && abs[3]<0){
        abs[1]=abs[1]*-1;
        abs[3]=abs[3]*-1;
    }
    
    if(abs[0]<0 && abs[2]<0){
        abs[0]=abs[0]*-1;
        abs[2]=abs[2]*-1;
    }
    
    if(abs[0]<abs[2]) X=abs[2]-abs[0];
    else X=abs[0]-abs[2];
    
    if(abs[1]<abs[3]) Y=abs[3]-abs[1];
    else Y=abs[1]-abs[3];
    
    printf("%f",(X+Y)/2);
    
}