望大佬解答为什么这样A了，另一个却WA了呢？//一个水题

#include<cstdio>
using namespace std;
#define rg register
typedef long long ll;
int n,m,a,b,xx[9]={0,-1,-2,-1,-2,1,2,1,2},yy[9]={0,-2,-1,2,1,2,1,-2,-1};
ll dp[22][22];
int main(void)
{
    scanf("%d%d%d%d",&n,&m,&a,&b);
    ++n;++m;++a;++b;
    for(rg int i=0;i<9;++i){
        int ii=a+xx[i],jj=b+yy[i];
        if(ii>=1&&ii<=n&&jj>=1&&jj<=m) dp[ii][jj]=-1;
    }
    dp[1][0]=1;
    for(rg int i=1;i<=n;++i)
        for(rg int j=1;j<=m;++j)
            dp[i][j]=dp[i][j]==-1?0:dp[i-1][j]+dp[i][j-1];
    printf("%lld",dp[n][m]);
}

// luogu-judger-enable-o2
#include<cstdio>
using namespace std;
#define rg register
typedef long long ll;
int n,m,a,b,xx[9]={0,-1,-2,-1,-2,1,2,1,2},yy[9]={0,-2,-1,2,1,2,1,-2,-1};
ll dp[33][33];
int main(void)
{
    scanf("%d%d%d%d",&n,&m,&a,&b);
    dp[0][0]=1;
    for(rg int i=0;i<9;++i){
        int ii=a+xx[i],jj=b+yy[i];
        if(ii>=0&&ii<=n&&jj>=0&&jj<=m) dp[ii][jj]=-1;
    }
    for(rg int i=1;i<=m;++i){
        if(dp[0][i]!=-1) dp[0][i]=1;
        else break;
    }
    for(rg int i=1;i<=n;++i){
        if(dp[i][0]!=-1) dp[i][0]=1;
        else break;
    }
    for(rg int i=1;i<=n;++i)
        for(rg int j=1;j<=m;++j)
            dp[i][j]=dp[i][j]==-1?0:dp[i-1][j]+dp[i][j-1];
    printf("%lld",dp[n][m]);
}