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

i = 0
count = 0
while i < N:
    if p_list[i] == i + 1:
        temp = p_list[i+1]
        p_list[i+1] = p_list[i]
        p_list[i] = temp
        count = count + 1
        i = i + 1
    i = i + 1

print(count)
