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

int main(){

    double x1,y1,x2,y2,x,y,d;

    scanf("%lf %lf %lf %lf",&x1,&y1,&x2,&y2);

    x = x2 - x1;
    y = y2 - y1;
    x = fabs(x);
    y = fabs(y);

    x = x * x;
    y = y * y;

    d = sqrt(x+y);

    printf("%lf\n", d);

    return 0;
}

