import sys

readline = sys.stdin.buffer.readline


def main():
    N = int(readline())
    P = list(map(int, readline().split()))
    P = list(map(lambda x: x-1, P))

    cnt = 0
    for i in range(0, N):
        if P[i] == i:
            if P[i+1] == i+1 and i != N-1:
                P[i], P[i+1] = P[i+1], P[i]
                cnt += 1
            elif i != 0:
                P[i], P[i-1] = P[i-1], P[i]
                cnt += 1
            else:
                cnt += 1

    print(cnt)


main()
