N = int(input())
P = list(map(int, input().split()))
Q = P.copy()
cnt1 = 0
for i in range(N-1):
    if P[i] == i+1:
        tmp = P[i]
        P[i] = P[i+1]
        P[i+1] = tmp
        cnt1 += 1
if P[i] == N:
    cnt1 += 1

cnt2 = 0
for i in range(N-1, 0, -1):
    if Q[i] == i+1:
        tmp = Q[i]
        Q[i] = Q[i-1]
        Q[i-1] = tmp
        cnt2 += 1
if Q[0] == 1:
    cnt2 += 1
print(min(cnt1, cnt2))
