#include<stdio.h>
#include<math.h>
int main()
{
    int  x1,y1,x2,y2,x,y;
    double d;
    scanf("%d %d %d %d",&x1,&y1,&x2,&y2);
    x=pow((x1-x2),2);
    y=pow((y1-y2),2);
    d=sqrt(x+y);
    printf("%.8lf\n",d);
    return 0;
}

