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

int main(int argc, const char * argv[]) {
    long double x1, x2, y1, y2, pow1, pow2, distance;
    scanf("%Lf %Lf %Lf %Lf", &x1, &y1, &x2, &y2);
    pow1 = powl((x1 - x2), 2);
    pow2 = powl((y1 - y2), 2);
    distance = sqrtl(pow1 + pow2);
    printf("%5Lf\n", distance);
    
    return 0;
}