#include using namespace std; typedef long long signed int LL; typedef long long unsigned int LU; #define incID(i, l, r) for(int i = (l) ; i < (r); i++) #define incII(i, l, r) for(int i = (l) ; i <= (r); i++) #define decID(i, l, r) for(int i = (r) - 1; i >= (l); i--) #define decII(i, l, r) for(int i = (r) ; i >= (l); i--) #define inc(i, n) incID(i, 0, n) #define inc1(i, n) incII(i, 1, n) #define dec(i, n) decID(i, 0, n) #define dec1(i, n) decII(i, 1, n) #define inII(v, l, r) ((l) <= (v) && (v) <= (r)) #define inID(v, l, r) ((l) <= (v) && (v) < (r)) #define PB push_back #define EB emplace_back #define MP make_pair #define FI first #define SE second #define UB upper_bound #define LB lower_bound #define PQ priority_queue #define ALL(v) v.begin(), v.end() #define RALL(v) v.rbegin(), v.rend() #define FOR(it, v) for(auto it = v.begin(); it != v.end(); ++it) #define RFOR(it, v) for(auto it = v.rbegin(); it != v.rend(); ++it) template bool setmin(T & a, T b) { if(b < a) { a = b; return true; } else { return false; } } template bool setmax(T & a, T b) { if(b > a) { a = b; return true; } else { return false; } } template bool setmineq(T & a, T b) { if(b <= a) { a = b; return true; } else { return false; } } template bool setmaxeq(T & a, T b) { if(b >= a) { a = b; return true; } else { return false; } } template T gcd(T a, T b) { return (b == 0 ? a : gcd(b, a % b)); } template T lcm(T a, T b) { return a / gcd(a, b) * b; } // ---- ---- LL n, a[2000]; LL dp[2][2001][2001]; const LL INF = 1000002000; int main() { cin >> n; inc(i, n) { cin >> a[i]; } inc(i, n + 1) { inc(j, n + 1) { dp[0][i][j] = dp[1][i][j] = INF; } } dp[0][0][n] = -1; dp[1][0][n] = -1; dec1(s, n) { inc(i, n - s + 1) { // cout << "[" << i << ", " << i + s << ") "; setmin(dp[0][i + 1][i + s], max(dp[0][i][i + s] + 1, a[i])); setmin(dp[0][i + 1][i + s], max(dp[1][i][i + s] + s, a[i])); setmin(dp[1][i][i + s - 1], max(dp[1][i][i + s] + 1, a[i + s - 1])); setmin(dp[1][i][i + s - 1], max(dp[0][i][i + s] + s, a[i + s - 1])); } // cout << endl; } /* inc(k, 2) { inc(i, n + 1) { inc(j, n + 1) { printf(" %3lld", dp[k][i][j] == INF ? -1 : dp[k][i][j]); } cout << endl; } cout << endl; } */ LL ans = INF; inc(i, n + 1) { setmin(ans, dp[0][i][i]); setmin(ans, dp[1][i][i]); } cout << ans << endl; return 0; }