N=int(input())
A=[int(x) for x in input().split()]
import heapq
q=[]
for i in range(N):
	heapq.heappush(q,(-A[i],i))
ans=0
while q:
	c,x=heapq.heappop(q)
	if x==0 or x==N-1:
		continue
	if A[x-1]<A[x] and A[x]>A[x+1]:
		A[x]=min(A[x-1],A[x+1])-1
		heapq.heappush(q,(-A[x],x))
		ans+=1
print(ans)