from heapq import heappush,heappop

n = int(input())
A = list(map(int,input().split()))
num = A[:]
done = [0]*n

h = []
for i,a in enumerate(A):
    heappush(h,[-a,i])

ans = 0
while h:
    c,ind = heappop(h)
    # print(c,ind)
    c = -c
    if ind == 0 or ind == n-1:
        done[ind] = 1   
        num[ind] = c
        continue

    if done[ind-1] == done[ind+1] == 0:
        ans += 1
        c = min(num[ind-1],num[ind+1])-1
        num[ind] = c
        heappush(h,[-c,ind])
    else:
        done[ind] = 1
print(ans)