样例过了,但$40$分,求助$!$

$P1605$ $迷宫$

#include<bits/stdc++.h>
using namespace std;
int dx[4]={0,1,-1,0};
int dy[4]={1,0,0,-1};
int r,c,k,sx,sy,lx,ly;
int vis[20][20],ans=0;
void dfs(int x,int y){
	if(x==lx&&y==ly){
		ans++;
		return ;
	}
	vis[x][y]=1;
	for(int i=0;i<2;i++){
		int nx=x+dx[i],ny=y+dy[i];
		if(nx<1||nx>r||ny<1||ny>c||vis[nx][ny]==1) continue;
		dfs(nx,ny);
	}
	vis[x][y]=0;
}
int main()
{
	int a,b;
	cin>>r>>c>>k>>sx>>sy>>lx>>ly;
	for(int i=1;i<=k;i++){
		cin>>a>>b;
		vis[a][b]=1;
	}
	vis[sx][sy]=1;
	dfs(sx,sy);
	cout<<ans;
 	return 0;
}