n = int(input())
p = list(map(int, input().split()))
ans = 0
for i in range(n-1):
    if p[i] == i+1:
        p[i], p[i+1] = p[i+1], p[i]
        ans += 1
if p[n-1] = n:
    p[n-1], p[n-2] = p[n-2], p[n-1]
    ans += 1
print(ans)