#include <stdio.h>
#include <string.h>
#include <math.h>
 
int main(void){
 
    int a, b, c, d;
    double length;
    
    scanf("%d %d %d %d", &a, &b, &c, &d);
    
    length = sqrt((a-c)*(a-c)+(b-d)*(b-d));
    
    printf("%f\n", length);
    
    return 0;
 
}