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