为啥会WA2个点?

#include<iostream>
using namespace std;
long long f[22][22];
bool b[22][22]={0};
int main()
{
	int n,m,x,y;
	cin>>n>>m>>x>>y;
	b[x][y]=1;
	b[x-2][y-1]=b[x-2][y+1]=b[x-1][y+2]=b[x-1][y-2]=1;
	b[x+1][y+2]=b[x+1][y-2]=b[x+2][y-1]=b[x+2][y+1]=1;
	for(int i=0;i<=n;i++) 
	{
		if(b[i][0]==0)
			f[i][0]=1;
	}
	for(int i=0;i<=m;i++) 
	{
		if(b[0][i]==0)
			f[0][i]=1;
	}
	for(int i=1;i<=n;i++)
	{
		for(int j=1;j<=m;j++)
		{
			if(b[i][j]==0)
				f[i][j]=f[i-1][j]+f[i][j-1];
			else
				f[i][j]=0;
		}
	}
	cout<<f[n][m];
	return 0;
}