为什么0分

#include<iostream>
#include<cstring>
using namespace std;
long long num=1,n,m,x,y;
bool cant[21][21];
void dfs(int a,int b)
{
	bool ok=false;
	if(a+1<=n&&!cant[a+1][b])
	{
		dfs(a+1,b);
		ok=true;
	}
	if(b+1<=m&&!cant[a][b+1])
	{
		if(ok)num++;
		dfs(a,b+1);
	}
	else if(a+1<n&&b+1<m)--num;
}
int main()
{
	cin>>n>>m>>x>>y;
	n+=2;
	m+=2;
	x+=2;
	y+=2;
	cant[x+1][y+2]=true;
	cant[x+2][y+1]=true;
	cant[x-2][y+1]=true;
	cant[x+1][y-2]=true;
	cant[x-1][y+2]=true;
	cant[x+2][y-1]=true;
	cant[x-2][y-1]=true;
	cant[x-1][y-2]=true;
	//cant[x][y]=true;
	dfs(2,2);
	cout<<num;
 }