#include "bits/stdc++.h"

#include <unordered_map>

//#include <iostream>//POJ
//#include <vector>
//#include <string>
//#include <iomanip>
//#include <math.h>
//#include <algorithm>
//#include <cstring>
using namespace std;
#define setc cin.tie(0);ios::sync_with_stdio(0)
#define dd(n) cout<<fixed<<setprecision(n)
#define inp(x) (cin>>x,x)
#define repi(i,a,b) for(int i=(a), i##_len=(b); i<i##_len; ++i)
#define rep(i,n) repi(i,0,n)
#define repi_(i,a,b) for(int i=(a), i##_len=(b); i<=i##_len; ++i)
#define rep_(i,n) repi_(i,0,n)
#define repir(i,a,b) for(int i=(a)-1, i##_first=(b); i>=i##_first; --i)
#define repr(i,n) repir(i,n,0)
#define repir_(i,a,b) for(int i=(a), i##_first=(b); i>=i##_first; --i)
#define repr_(i,n) repir_(i,n,0)
#define all(x) (x).begin(),(x).end()
#define vsort(v) sort((v).begin(), (v).end())
#define gsort(v) tsort(v); reverse((v).begin(), (v).end())
#define vrev(v) reverse((v).begin(), (v).end())
//template<typename T> T vpop(vector<T> *v) {
//	T res = v->back();
//	v->pop_back();
//	return res;
//}
//char vpop(string *v) {
//	char res = v->back();
//	v->pop_back();
//	return res;
//}
#define siz(v) ((int)(v).size())
#define ers(v, n) (v).erase((v).begin() + n)
#define cnt(v, n) count(all(v), n)
#define vmin(v) *min_element(v.begin(), v.end())
#define vmax(v) *max_element(v.begin(), v.end())
#define contain(q) !q.empty()
#define cont(q) !q.empty()
//#define qpop(q, a, b) a=q.back().first;b=q.back().second;q.pop()
//#define pqpop(p, a, b) a=q.top().first;b=q.top().second;q.pop()
#define el "\n"
#define sp " "
#define pi 3.14159265358979
#define co(x) cout<<x<<el
#define coc(c, a, b) if(c)co((a));else co((b))
#define cosp(x) cout<<(x)<<' '
#define YES(c) coc(c,"YES", "NO")
#define Yes(c) coc(c,"Yes", "No")
#define yes(c) coc(c,"yes", "no")
#define POSSIBLE(c) coc(c, "POSSIBLE", "IMPOSSIBLE")
#define Possible(c) coc(c, "Possible", "Impossible")
#define possible(c) coc(c, "possible", "impossible")
#define inf INT_MAX
#define wildcard(T) numeric_limits<T>::min()

#define pb push_back
#define pq priority_queue
#define np next_permutation

#ifdef tqktmp_2
#define gc() getchar(); getchar()
#else
#define gc() 1
#endif
const unsigned int bf0 = (1 << 0);
const unsigned int bf1 = (1 << 1);
const unsigned int bf2 = (1 << 2);
const unsigned int bf3 = (1 << 3);
const unsigned int bf4 = (1 << 4);
const unsigned int bf5 = (1 << 5);
const unsigned int bf6 = (1 << 6);
const unsigned int bf7 = (1 << 7);

//#define lint long long
typedef long long lint;
typedef vector<int> ivec; typedef vector<string> svec;
typedef vector<lint> livec;
typedef vector<vector<int>> ivvec;
typedef pair<int, int> P;

const int
dx8[8] ={ 0,1,1,1,0,-1,-1,-1 },
dy8[8] ={ 1,1,0,-1,-1,-1,0,1 },
dx9[9] ={ 0,1,1,1,0,0,-1,-1,-1 },
dy9[9] ={ 1,1,0,-1,0,-1,-1,0,1 },
dx4[4] ={ 0,1,0,-1 },
dy4[4] ={ 1,0,-1,0 };

template<class T>inline bool maxi(T &a, const T &b) { if (a < b) { a = b; return 1; } return 0; }
template<class T>inline bool mini(T &a, const T &b) { if (b < a) { a = b; return 1; } return 0; }

#define mod(a,b) (a<0?a%b+abs(b):a%b)

int main(){
	setc;

	lint n,in,ans=0;cin>>n;
	livec a(n);
	bool f=0;
	rep(i, n){
		cin>>a[i];
	}rep(i, n){
		if (a[i]==i+1){
			f=1;
			if(i+1<n)swap(a[i],a[i+1]);
		}
		else f=0;
		if(f)++ans;
	}co(ans);
	gc();
}

//int main(){
//	setc;
//
//	lint q, h, s, d, n; cin>>q>>h>>s>>d>>n;
//	lint one=min({4*q,2*h,s});
//	lint two=min(2*one,d);
//	if(n==1)co(one);
//	else{
//		co(n/2*two+n%2*one);
//	}
//	gc();
//}

//
//
//lint dp[10]={}; lint dppos=0;
//
//int main(){
//	setc;
//
//	lint q, h, s, d, n; cin>>q>>h>>s>>d>>n; q*=4, h*=4, s*=4, d*=4, n*=4;
//	repi_(i, 1, n){
//		dp[i%10]=dp[(i-1)%10]+q; if (i<2)continue;
//		dp[i%10]=min(dp[i%10], dp[(i-2)%10]+h); if (i<4)continue;
//		dp[i%10]=min(dp[i%10], dp[(i-4)%10]+s); if (i<8)continue;
//		dp[i%10]=min(dp[i%10], dp[(i-8)%10]+d);
//	}co(dp[n%10]/4);
//	gc();
//}


Measured memory_bytes: 1024