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

int main(){
    long a_x, a_y, b_x, b_y;
    scanf("%ld %ld %ld %ld", &a_x, &a_y, &b_x, &b_y);
    double time_x, time_y;
    time_x = labs(((double)(a_x - b_x) / 2));
    time_y = labs(((double)(a_y - b_y) / 2));
    if(time_x < time_y){
        printf("%d\n", time_x);
    }else{
        printf("%d\n", time_y);
    }
    return 0;
}