#include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include using namespace std; #define sz size() #define pb push_back #define mp make_pair #define fi first #define se second #define all(c) (c).begin(), (c).end() #define rep(i,a,b) for(int i=(a);i<(b);++i) #define clr(a, b) memset((a), (b) ,sizeof(a)) #define MOD 1000000009 int dp[3000][3000][2]; int main(){ int n; cin>>n; if(n==1){ cout << 1 << endl; return 0; } vector v; rep(i,0,n){ int a; cin>>a; v.pb(a); } clr(dp,-1); rep(i,0,3000){ dp[i][i][0] = 1; dp[i][i][1] = 1; } rep(i,2,n+1){ rep(j,0,n-i+1){ // left { int l = j+1; int r = j+i-1; if(dp[l][r][1]!=-1&&v[j]