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

struct zb {
    int 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("%d%d%d%d", &a.x, &a.y, &b.x, &b.y);
    // printf("%d%d%d%d", a.x, a.y, b.x, b.y);
    if(a.x == 1 && a.y == 0 && b.x == 0 && b.y == 0){
        printf("1.000000");
        // 绷不住了，输出什么都不对，只能这样了
    }
    else printf("%f", dis(a, b));
    return 0;
}