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

int main(void){
    double x1, y1, x2, y2;
    double distance;
    
    scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);
    
    distance = sqrt( pow(x1-x2, 2) + pow(y1-y2, 2) );

    printf("%.8f", distance);    
    
    return 0;
}

