def main():
    import sys
    input = sys.stdin.read
    data = input().split()
    
    N = int(data[0])
    A = list(map(int, data[1:N+1]))
    
    INF = float('inf')
    left = [[INF] * (N + 2) for _ in range(N + 2)]
    right = [[INF] * (N + 2) for _ in range(N + 2)]
    
    for i in range(1, N + 1):
        left[i][i] = A[i - 1]
        right[i][i] = A[i - 1]
    
    for length in range(1, N):
        for l in range(1, N - length + 1):
            r = l + length
            
            # Expand from left end of [l, r-1] to right
            current_left = left[l][r - 1]
            if current_left != INF:
                time = current_left + (r - l)
                required = max(time, A[r - 1])
                if required < right[l][r]:
                    right[l][r] = required
            
            # Expand from right end of [l, r-1] to right
            current_right = right[l][r - 1]
            if current_right != INF:
                time = current_right + 1
                required = max(time, A[r - 1])
                if required < right[l][r]:
                    right[l][r] = required
            
            # Expand from right end of [l+1, r] to left
            current_right_sub = right[l + 1][r]
            if current_right_sub != INF:
                time = current_right_sub + (r - l)
                required = max(time, A[l - 1])
                if required < left[l][r]:
                    left[l][r] = required
            
            # Expand from left end of [l+1, r] to left
            current_left_sub = left[l + 1][r]
            if current_left_sub != INF:
                time = current_left_sub + 1
                required = max(time, A[l - 1])
                if required < left[l][r]:
                    left[l][r] = required
    
    print(min(left[1][N], right[1][N]))

if __name__ == "__main__":
    main()