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n = int(input())
entry = input().split()
entry = list(map(int, entry))
counter = 0
for i in range(1, n-1, 2):
    if entry[i]>=entry[i+1] or entry[i]>=entry[i-1]:
        entry[i] = min(entry[i+1], entry[i-1])-1
        counter+=1
if entry[n-1]>=entry[n-2] and entry[n-1]%2==0:
    entry[n-1]=entry[n-2]-1
    counter+=1
print(counter)