/***************************************
    codeforces = topcoder = sahedsohel
    IIT,Jahangirnagar University(42)
****************************************/
#include <bits/stdc++.h>
using namespace std;

#define ll long long int
#define ull unsigned long long int
#define inf (INT_MAX/10)
#define linf (LLONG_MAX/10LL)
#define sc(a) scanf("%d",&a)
#define sc2(a,b) scanf("%d%d",&a,&b)
#define sc3(a,b,c) scanf("%d%d%d",&a,&b,&c)
#define sc4(a,b,c,d) scanf("%d%d%d%d",&a,&b,&c,&d)
#define fl(c,i,n) for(i=c;i<n;i++)
#define f(i,n) for(i=0;i<n;i++)
#define mem(a) memset(a,0,sizeof(a))
#define memn(a) memset(a,-1,sizeof(a))
#define pb push_back
#define aov(a) a.begin(),a.end()
#define mpr make_pair
#define PI (2.0*acos(0.0)) //#define PI acos(-1.0)
#define xx first
#define yy second
#define mxv(a) *max_element(aov(a))
#define mnv(a) *min_element(aov(a))
#define LB(a,x) (lower_bound(aov(a),x)-a.begin())
#define UB(a,x) (upper_bound(aov(a),x)-a.begin())
#define to_c_string(a) a.c_str()
#define strtoint(c) atoi(&c[0])
#define pll pair< ll , ll >
#define pii pair< int , int >
#define pcs(a) printf("Case %d: ", a)
#define nl puts("")
#define endl '\n'
#define dbg(x) cout<<#x<<" : "<<x<<endl

template <class T> inline T bigmod(T p,T e,T M){ll ret = 1;for(; e > 0; e >>= 1){if(e & 1) ret = (ret * p) % M;p = (p * p) % M;}return (T)ret;}
template <class T> inline T gcd(T a,T b){if(b==0)return a;return gcd(b,a%b);}
template <class T> inline T modinverse(T a,T M){return bigmod(a,M-2,M);}   // M is prime}
template <class T> inline T bpow(T p,T e){ll ret = 1;for(; e > 0; e >>= 1){if(e & 1) ret = (ret * p);p = (p * p);}return (T)ret;}

int toInt(string s){int sm;stringstream ss(s);ss>>sm;return sm;}
int toLlint(string s){long long int sm;stringstream ss(s);ss>>sm;return sm;}


///int mnth[]={-1,31,28,31,30,31,30,31,31,30,31,30,31};  //Not Leap Year
///int dx[]={2,1,-1,-2,-2,-1,1,2};int dy[]={1,2,2,1,-1,-2,-2,-1};//Knight Direction
///int dx[]={-1,+1,0,1,0,-1}; // Hexagonal Direction   **
///int dy[]={-1,+1,1,0,-1,0}; //                       *#*
///                                                     **
///const double eps=1e-9;

/*****************************************************************/
/// ////////////////////   GET SET GO    ////////////////////// ///
/*****************************************************************/


#define intx(i,j,k,l) ((a[i]*b[j]-b[i]*a[j])*(a[k]-a[l])-(a[i]-a[j])*(a[k]*b[l]-b[k]*a[l]))/((a[i]-a[j])*(b[k]-b[l])-(b[i]-b[j])*(a[k]-a[l]))
#define inty(i,j,k,l) ((a[i]*b[j]-b[i]*a[j])*(b[k]-b[l])-(b[i]-b[j])*(a[k]*b[l]-b[k]*a[l]))/((a[i]-a[j])*(b[k]-b[l])-(b[i]-b[j])*(a[k]-a[l]))
#define dst(u,v,x,y) sqrt((x*1.0-u*1.0)*(x*1.0-u*1.0)+(y*1.0-v*1.0)*(y*1.0-v*1.0))
#define area(p1,p2,p3) (p1.xx*p2.yy+p2.xx*p3.yy+p3.xx*p1.yy-p1.yy*p2.xx-p2.yy*p3.xx-p3.yy*p1.xx)
#define dstt(u,v,x,y) ((ll)(x-u)*(ll)(x-u)+(ll)(y-v)*(ll)(y-v))

int ts,kk=1;

#define sqr(x) ((x)*(x))

/// 	   0123456789ABCDEF
#define M  100005
#define MD 1000000007LL
#define MX 1000005
#define mid ((s+e)>>1)
#define lft ((i<<1)+1)
#define rgt (lft+1)

int n;
int a[M],b[M];

int main()
{
    int t,i,j,k;
    
    
    
    sc(n);
    fl(1,i,n+1)sc(a[i]),b[i]=a[i];
	
	int rs=0;
	fl(1,i,n+1)
	{
		if(a[i]==i)
		swap(a[i],a[i+1]),rs++;
	}
	//if(a[n]==n)rs++;
	int r2=0;
	for(i=n;i>0;i--)
	{
		if(b[i]==i)
		swap(b[i],b[i-1]),r2++;
	}
	//if(b[1]==1)r2++;
	
	printf("%d\n",min(rs,r2));
    
    
	return 0;
}


Measured memory_bytes: 1024