#include <bits/stdc++.h>
#include "grader.h"
using namespace std;


int rectangle(int R, int C, int H, int W, int Q[3001][3001]) {
    return R*C/2;
}

static int R,C,H,W,Q[3001][3001],i,j,ans;
int _main(rectangleFunction rectangle){
   scanf("%d%d%d%d",&R,&C,&H,&W);
   for (i=0;i<R;i++) for (j=0;j<C;j++) scanf("%d",&Q[i][j]);
   ans = rectangle(R,C,H,W,Q);
   printf("%d\n",ans);
   return 0;
}


int main(){
    return _main(rectangle);
}