4TLE，实在想不出记忆化搜索怎么写好

#include <iostream>
using namespace std;
int n,m;
const int N = 1e7;
int dp[N] = {0,};
int sum = 0;
int solve(int num,int m1){
	if(m1<m){
		if(m1==m-1){
			m1++;
		    if(num-1==1||num+1>n){
			    sum++;
	    }
	}else{
		m1++;
		if(num-1==0){
			solve(n,m1);
			solve(num+1,m1);
		}else if(num+1>n){
		    solve(1,m1);
		    solve(num-1,m1);
		}else{
			solve(num-1,m1);
			solve(num+1,m1);
			}	
		}
	}	
}
int main(){
	cin >> n >> m;
	solve(1,0);
	cout << sum;
	return 0;
}