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

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