import sys
import heapq
from itertools import permutations
from bisect import bisect_left, bisect_right
from collections import Counter, deque
from fractions import gcd
from math import factorial, sqrt
INF = 1 << 60
mod = 1000000007
sys.setrecursionlimit(10 ** 7)
input = sys.stdin.readline

# UnionFind
class UnionFind():
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n

    def find(self, x):
        if self.parents[x] < 0:
            return x
        else:
            self.parents[x] = self.find(self.parents[x])
            return self.parents[x]

    def union(self, x, y):
        x = self.find(x)
        y = self.find(y)

        if x == y:
            return

        if self.parents[x] > self.parents[y]:
            x, y = y, x

        self.parents[x] += self.parents[y]
        self.parents[y] = x

    def size(self, x):
        return -self.parents[self.find(x)]

    def same(self, x, y):
        return self.find(x) == self.find(y)

    def members(self, x):
        root = self.find(x)
        return [i for i in range(self.n) if self.find(i) == root]

    def roots(self):
        return [i for i, x in enumerate(self.parents) if x < 0]

    def group_count(self):
        return len(self.roots())

    def all_group_members(self):
        return {r: self.members(r) for r in self.roots()}

    def __str__(self):
        return '\n'.join('{}: {}'.format(r, self.members(r)) for r in self.roots())

# ダイクストラ
def dijkstra_heap(s, edge, n):
    #始点sから各頂点への最短距離
    d = [10**20] * n
    used = [True] * n #True:未確定
    d[s] = 0
    used[s] = False
    edgelist = []
    for a,b in edge[s]:
        heapq.heappush(edgelist,a*(10**6)+b)
    while len(edgelist):
        minedge = heapq.heappop(edgelist)
        #まだ使われてない頂点の中から最小の距離のものを探す
        if not used[minedge%(10**6)]:
            continue
        v = minedge%(10**6)
        d[v] = minedge//(10**6)
        used[v] = False
        for e in edge[v]:
            if used[e[1]]:
                heapq.heappush(edgelist,(e[0]+d[v])*(10**6)+e[1])
    return d

#　素因数分解
def factorization(n):
    arr = []
    temp = n
    for i in range(2, int(-(-n**0.5//1))+1):
        if temp%i==0:
            cnt=0
            while temp%i==0:
                cnt+=1
                temp //= i
            arr.append([i, cnt])

    if temp!=1:
        arr.append([temp, 1])

    if arr==[]:
        arr.append([n, 1])

    return arr

# ここから書き始める
n = int(input())
p = list(map(int, input().split()))
q = [0] * n
for i in range(n):
    if p[i] == i + 1:
        q[i] = 1
# print(q)
ans = 0
for i in range(1, n - 1):
    if q[i] == 0:
        continue
    ans += 1
    # if i == 0:
    #     if q[1] == 1:
    #         p[0], p[1] = p[1], p[0]
    #         q[0] = 0
    #         q[1] = 0
    #     continue
    # if i == n - 1:
    #     break
    if i != n - 2:
        if p[i - 1] != i + 1:
            p[i - 1], p[i] = p[i], p[i - 1]
            q[i] = 0
            q[i - 1] = 0
        else:
            p[i], p[i + 1] = p[i + 1], p[i]
            q[i] = 0
            q[i + 1] = 0
    else:
        if p[i + 1] != i + 1:
            p[i], p[i + 1] = p[i + 1], p[i]
            q[i] = 0
            q[i + 1] = 0
        else:
            p[i - 1], p[i] = p[i], p[i - 1]
            q[i - 1] = 0
            q[i] = 0
if p[0] == 1 or p[n - 1] == n:
    ans += 1
print(ans)