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

int main(){
    long double x1, y1, x2, y2, distance;
    scanf("%Lf %Lf %Lf %Lf", &x1, &y1, &x2, &y2);

    distance = sqrt(pow(x2 - x1, 2) + pow(y2 - y1, 2));

    printf("%Lf\n", distance);
    return 0;
}
