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