#include <cmath>
#include <iostream>
using namespace std;

int main() {
    int n = 2;
    double x1, y1, x2, y2;
        cin >> x1 >> y1 >> x2 >> y2;
        double c = sqrt((x1 - x2)*(x1 - x2) + (y1 - y2) * (y1 - y2));
        double d = fabs(x1 - x2) + fabs(y1 - y2);
        double e = fabs(d - c);
        printf("%.9lf", e);

    return 0;
    
}
// 64 位输出请用 printf("%lld")