为什么递归会超时

#include<iostream>
using namespace std;
long long num=0;int chess[21][21],n,m,x,y;
int dx[9]={0,1,2,1,2,-1,-2,-1,-2};
int dy[9]={0,-2,-1,2,1,-2,-1,2,1};
void read()
{
    int i;
    cin>>n>>m>>x>>y;
    chess[x][y]=1;
    for (i=1;i<=8;i++)
    {
        if (x+dx[i]>=1&&x+dx[i]<=n&&y+dy[i]>=1&&y+dy[i]<=m) 
        chess[x+dx[i]][y+dy[i]]=1;
    }
}
void fang(int x,int y)
{
    if (x==n&&y==m)
    {
        num++;
        return;
    }
    if (chess[x][y+1]==0&&y+1<=m)
    fang(x,y+1);
    if (chess[x+1][y]==0&&x+1<=n)
    fang(x+1,y);
}
int main()
{
    read();
    fang(0,0);
    cout<<num;
     return 0;
}