#include <stdio.h>
#include <assert.h>

int assess(int A, int B, int X, int Y)
{
    int mag1,mag2, min1, min2, ris;
    if (A>=B)
    {
       mag1=A;
       min1=B;
    }
    else
    {
       mag1=B;
       min1=A;
    }
    if (X>=Y)
    {
       mag2=X;
       min2=Y;
    }
    else
    {
       mag2=Y;
       min2=X;
    }
    if(mag1>=mag2 && min1>=min2)
                  return 0;
    if(mag1<mag2 && min1<min2)
        return((X*Y)-(A*B));
    if(mag1>=mag2)
    {
        if ((min2-min1)%2==0)
           ris=(min2-min1)/2*mag2;
        else
           ris=((min2-min1)/2+1)*mag2;
    }
        if(min1>=min2)
    {
        if ((mag2-mag1)%2==0)
           ris=(mag2-mag1)/2*min2;
        else
           ris=((mag2-mag1)/2+1)*min2;
    }
    return ris;
}


int main() {
    FILE *fr, *fw;
    int A, B, X, Y;

    fr = fopen("input.txt", "r");
    fw = fopen("output.txt", "w");
    assert(4 == fscanf(fr, "%d %d %d %d", &A, &B, &X, &Y));

    fprintf(fw, "%d\n", assess(A, B, X, Y));
    fclose(fr);
    fclose(fw);
    return 0;
}