def main():
    N = int(input())
    p = [0] + list(map(int, input().split()))
    
    ans = 0
    for i in range(1, N):
        if i == p[i] or i+1 == p[i+1]:
            p[i], p[i+1] = p[i+1], p[i]
            ans += 1
    
    print(ans)

if __name__ == '__main__':
    main()    