大神求助……深搜问题

#include<iostream>
using namespace std;

int bx,by,n,m,book[21][21],count=0;

void dfs(int x,int y,int step)
{
	int next[2][2] = {{0,1},//向右走 
	                  {1,0}};//向下走 
	int tx,ty,k;
	if (x==bx&&x==by)
	{
		count++;
		return;
	}
	for(k=0;k<=1;k++)
	{
		tx=x+next[k][0];
		ty=y+next[k][1];//即下一步的 
		if(tx>20||ty>20)//判定是否出界 
			continue;
		if(book[tx][ty]==0)
		{
			dfs(tx,ty,step+1);
		}
		
	}
	return;
}

int main()
{
	cin>>bx>>by>>n>>m;
	book[n][m]=1;
	book[n-2][m-1]=1;
	book[n-2][m+1]=1;
	book[n-1][m-2]=1;
	book[n-1][m+2]=1;
	book[n+2][m+1]=1;
	book[n+2][m-1]=1;
	book[n+1][m+2]=1;
	book[n+1][m-2]=1;//马可以吃的区域 
	dfs(0,0,0);
	cout<<count;
	
	return 0;
}