#include <stdio.h>
#include <math.h>

int main(void){
double x1,y1,x2,y2;
scanf("%lf%lf%lf%lf",&x1,&y1,&x2,&y2);
printf("%.8lf\n",(hypotl((x2-x1),(y2-y1))));//%lf wo %ld to machigaete hamatta
//hypot seido ga tarinaitame dame 
//hpotf ha dame. dakedo 8bunno5ha seikai
//itiban seido no takai hypotl de tamesu
//hypotl ha 8bunno0 de mattakudame. hypotl ha support site inai mitai.
//hypot to hypotf madeha support siteiru.
//sokode sqrt de kakushika nai.
return 0;
}