n = int(input())
p = list(map(int, input().split()))
ans = 0

for i in range(n-1):
    if i+1 == p[i]:
        p[i], p[i+1] = p[i+1], p[i]
        ans += 1

if p[n-1] == n-1:
    ans += 1

print(ans)
        