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


