#include <cstdio>
double abs(int a){
    return (a<0?-a:a);
}
int main(){
    int px,py,qx,qy;
    scanf("%d %d %d %d",&px,&py,&qx,&qy);
    printf("%lf",(abs(px-qx)+abs(py-qy))/2);
    return 0;
}