n = int(input())
a = list(map(int, input().split()))
INF = float('inf')

# Precompute the maximum value in each interval [l..r]
max_a = [[0] * (n + 2) for _ in range(n + 2)]
for l in range(1, n + 1):
    current_max = a[l - 1]
    max_a[l][l] = current_max
    for r in range(l + 1, n + 1):
        current_max = max(current_max, a[r - 1])
        max_a[l][r] = current_max

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(2, n + 1):
    for l in range(1, n - length + 2):
        r = l + length - 1
        # Compute left[l][r]
        option1 = left[l + 1][r] + 1
        option1 = max(option1, max_a[l][r])
        option2 = right[l + 1][r] + (r - l)
        option2 = max(option2, max_a[l][r])
        left[l][r] = min(option1, option2)
        # Compute right[l][r]
        option1 = left[l][r - 1] + (r - l)
        option1 = max(option1, max_a[l][r])
        option2 = right[l][r - 1] + 1
        option2 = max(option2, max_a[l][r])
        right[l][r] = min(option1, option2)

result = min(left[1][n], right[1][n])
print(result)