import sys

stdin = sys.stdin

ni = lambda: int(ns())
na = lambda: list(map(int, stdin.readline().split()))
ns = lambda: stdin.readline().rstrip()

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