60分(TLE #3 #4)求调

#include<bits/stdc++.h>
using namespace std;
int n,m,x,y,ans=0;
bool vis[25][25];
void dfs(int a,int b){
	if(a>n||b>m||vis[a][b]||a<0||b<0) return;
	if(a==n&&b==m){
		ans++;
		return;
	}
	dfs(a+1,b);
	dfs(a,b+1);
}
int main(){
    cin>>n>>m>>x>>y;
    vis[x][y]=1;
	vis[x+1][y+2]=1;
	vis[x+1][y-2]=1;
	vis[x-1][y-2]=1;
	vis[x-1][y+2]=1;
	vis[x-2][y+1]=1;
	vis[x-2][y-1]=1;
	vis[x+2][y+1]=1;
	vis[x+2][y-1]=1;
	dfs(0,0);
	cout<<ans;
    return 0;
}