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

void distance(double x1,double y1,double x2,double y2) {
    double dis = sqrt ( (x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 -y2) );
    printf ("%f\n",dis);
}

int main () {
    double x1,y1,x2,y2;
    scanf ("%lf %lf %lf %lf",&x1,&y1,&x2,&y2);
    distance (x1,y1,x2,y2);
    
    return 0;
}
