#include <math.h>
#include <stdio.h>
#define square(x) ((x) * (x))

int main() {
    int x1, y1, x2, y2;
    scanf("%d %d %d %d", &x1, &y1, &x2, &y2);
    printf("%f", sqrt(square(x2 - x1) + square(y2 - y1)));
}