#include <bits/stdc++.h>
using namespace std;
#define int              long long int
#define double           long double
#define print(a)         for(auto x : a) cout << x << " "; cout << endl


const int M = 1000000007;
const int N = 3e5+9;
const int INF = 2e9+1;
const int LINF = 2000000000000000001;

inline int power(int a, int b, int mod=M) {
    int x = 1;
    a %= mod;
    while (b) {
        if (b & 1) x = (x * a) % mod; 
        a = (a * a) % mod;
        b >>= 1;
    }
    return x;
}


//_ ***************************** START Below *******************************




vector<int> a;

int consistency(int n){
	
	if(n==1) return 0;
	if(n==2) return 1;

	vector<vector<vector<int>>> dp(n, vector<vector<int>>(n, vector<int>(3, 0)));
	
	int s1 = a[0] + a[1];
	int s2 = a[0] + a[n-1];
	int s3 = a[n-2] + a[n-1];
	
	int target[3] = {s1, s2, s3};
	
	for(int k=0; k<3; k++){
		
	    for(int gap=0; gap<n; gap++){
			for(int i=0, j=gap; j<n; i++, j++){
				if(gap == 0){
					dp[i][j][k] = 0;
				}
				else if(gap == 1){
					
					if(a[i] + a[i+1] == target[k]){
						dp[i][j][k] = 1;
					}
				}
				else{
					
					if(a[i] + a[i+1] == target[k]){
						dp[i][j][k] = max( dp[i][j][k] , 1 + dp[i+2][j][k] );
					}
					if(a[i] + a[j] == target[k]){
						dp[i][j][k] = max( dp[i][j][k] , 1 + dp[i+1][j-1][k] );
					}
					if(a[j-1] + a[j] == target[k]){
						dp[i][j][k] = max( dp[i][j][k] , 1 + dp[i][j-2][k] );
					}
				}
			}
		}
	}

	return max({dp[0][n-1][0], dp[0][n-1][1], dp[0][n-1][2]});
}















int practice(int n){


    return 0;
}





void solve() {
    
    int n;
    cin>> n;
    
    a.resize(n);
    for(int i=0; i<n; i++) cin >> a[i];
    
    cout << consistency(n) << endl;


}





int32_t main() {
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);

    int t = 1;
    // cin >> t;
    while (t--) {
        solve();
    }

    return 0;
}