from collections import deque

N=int(input())
A=list(map(int,input().split()))

S=set([i for i in range(1,N-1) if A[i-1]<A[i] and A[i]>A[i+1]])

X=0
while S:
    i=S.pop()
    X+=1
    A[i]=min(A[i-1],A[i+1])-1

    if (i>=2) and A[i-2]<A[i-1] and A[i-1]>A[i]:
        S.add(i-1)

    if i<=N-3 and A[i]<A[i+1] and A[i+1]>A[i+2]:
        S.add(i+1)

print(X)