#include <stdio.h>
#include<math.h>
int main(void){
    double x1, x2, y1, y2;
    double z;
    scanf("%lf %lf %lf %lf\n", &x1, &y1, &x2, &y2);
    z = (x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1);
    printf("%.8lf\n", sqrt(z));
    
    return (0);
}
