import sys
def main():
    N = int(sys.stdin.readline())
    p = list(map(int, input().split()))
    count = 0
    for i, n in enumerate(p, 1):
        if i == n:
            try:
              	count += 1
                p[i - 1], p[i] = p[i], p[i - 1]
            except:
                break
    print(count)
 
if __name__ == '__main__':
    main()