#include <stdio.h>
#include<math.h>
int main() {
    int a,b,c,d;
    scanf("%d %d",&a,&b);
    scanf("%d %d",&c,&d);

    int x=a-c;
    int y=b-d;

    double de = sqrt(x*x+y*y);
    double dm = abs(x)+abs(y);
    double delta = fabs(dm-de);

    printf("%.12f\n",delta);
    return 0;
}