import sys
input = sys.stdin.readline
from heapq import heappop,heappush
from operator import itemgetter

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

H=[]

for i in range(N):
    H.append((-A[i],i))

H.sort(key=itemgetter(0))

ANS=0

while H:
    w,ind=heappop(H)

    if 1<=ind<N-1 and A[ind]>A[ind-1] and A[ind]>A[ind+1]:
        A[ind]=min(A[ind-1],A[ind+1])-1
        ANS+=1
        heappush(H,(-A[ind],ind))

print(ANS)