#include <cstdio>
#include <cmath>

int main() {
    double x1, y1, x2, y2;
    double dis;
    scanf("%lf%lf%lf%lf", &x1, &y1, &x2, &y2);

    dis = sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
    printf("%.4lf", dis);
    return 0;
}