#include <stdio.h>
#include <stdlib.h>

int main(){
    int Px, Py, Qx, Qy;
    double t;

    scanf("%d %d\n", &Px, &Py);
    scanf("%d %d\n", &Qx, &Qy);

    t = (double)abs(Px - Qx)/2.0 + (double)abs(Py - Qy)/2.0;
    printf("%lf\n", t);

    return 0;
}