3，4点超时，递归求优化！

#include<iostream>
#include<math.h>
using namespace std;
int a,b,n,m;
long long count;
int s[23][23];
void solve(int x,int y){
	if(x==a&&y==b){
		count++;
		return;
	}
	if(x==a+1||y==b+1)return;
	if(s[x+1][y]==1&&s[x][y+1]==1)return;
	if(s[x][y]==1)return;
	solve(x+1,y);
	solve(x,y+1);
}
int main(){
	cin>>a>>b>>n>>m;
	s[n][m] = 1;
if(n-1>=0&&m+2<=b)	s[n-1][m+2] = 1;
if(n-1>=0&&m-2>=0)	s[n-1][m-2] = 1;
if(n+1<=a&&m+2<=b)	s[n+1][m+2] = 1;
if(n+1<=a&&m-2>=0)	s[n+1][m-2] = 1;
if(n-2>=0&&m+1<=b)	s[n-2][m+1] = 1;
if(n-2>=0&&m-1>=0)	s[n-2][m-1] = 1;
if(n+2<=a&&m+1<=b)	s[n+2][m+1] = 1;
if(n+2<=a&&m-1>=0)	s[n+2][m-1] = 1;
	solve(0,0);	
	cout<<count;
}