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

int main(){
    double x1 , x2 , y1 ,y2;
    double dis=0;
    
    scanf("%lf %lf %lf %lf",&x1,&y1,&x2,&y2);
    dis=((x1 - x2)*(x1 - x2)+(y1 - y2)*(y1 - y2));
    dis= sqrt(dis);
    printf("%f\n",dis);
    
return 0;
}