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

struct zb {
    float x, y;
} a, b;

float dis(struct zb a, struct zb b) {
    return sqrt((a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y));
}

int main() {
    scanf("%f%f%f%f", &a.x, &a.y, &b.x, &b.y);
    printf("%f", dis(a, b));
    return 0;
}