//Distance 
//?????¢
#include<stdio.h>
#include<math.h>


int main(void){
    double x1, x2;
    double y1, y2;
    double distance;
    
    scanf("%lf", &x1); scanf("%lf", &y1);
    scanf("%lf", &x2); scanf("%lf", &y2);
    
    distance = sqrt(pow(x2 - x1, 2.0) + pow(y2-y1, 2.0));
    printf("%lf\n", distance);
    
    return 0;
}