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

void distance(double *x1, double *y1, double *x2, double *y2){
    double dis;
    dis = pow(*x1-*x2,2) + pow(*y1-*y2,2);
    dis = sqrt(dis);
    printf("%f\n", dis);
}

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