/*input
5
1 4 3 5 2
*/
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

#define MOD 1000000007

#define MODSET(d) if ((d) >= MOD) d %= MOD;
#define MODNEGSET(d) if ((d) < 0) d = ((d % MOD) + MOD) % MOD;
#define MODADDSET(d) if ((d) >= MOD) d -= MOD;
#define MODADDWHILESET(d) while ((d) >= MOD) d -= MOD;



//defines   

#define dsc1(a,type) type a; cin>>a;
#define dsc2(a,b,type) type a,b; cin>>a>>b;
#define dsc3(a, b, c,type) type a,b,c; cin>>a>>b>>c;
#define dsc4(a, b, c, d,type) type a ,b,c,d; cin>>a>>b>>c>>d;
#define nl    cout<<"\n";
#define foreach(v, c) for(__typeof( (c).begin()) v = (c).begin();  v != (c).end(); ++v)
#define revforeach(v, c) for(__typeof( (c).rbegin()) v = (c).rbegin();  v != (c).rend(); ++v)

#define fastio ios_base::sync_with_stdio(0);cin.tie(0);
#define re(i,b) for(int i=0;i<int(b);i++)
#define re1(i,b) for(int i=1;i<=int(b);i++)
#define all(c) c.begin(), c.end() 
#define rall(c) c.rbegin(),c.rend()
#define mpresent(container, element) (container.find(element) != container.end()) //for map,set..etc (returns true/false value)
#define vpresent(container, element) (find(all(container),element) != container.end()) //for vectors,strings,list,deque (returns true/false value)

#define eb emplace_back
#define mp make_pair
#define fi first
#define se second
#define pb push_back
#define pf push_front
#define ins insert
#define F first
#define S second
#define clr clear()
#define sqr(x) ((x)*(x))
#define sz(x) ((int)x.size())
#define dt distance 
#define test(t) int t; cin>>t; while(t--)
#define csb(i) __builtin_popcount(i)
#define csbll(i) __builtin_popcountll(i)
#define clz(x) __builtin_clz(x)
#define clzl(x) __builtin_clzl(x)
#define cp(x) __builtin_parity(x)
#define adv(v,num) advance(v,num)//used for lists and other structures that use iterators,when you can't access elements randomly ( iterator moves num positions)
#define mod 1000000007
#define MAX_ARR 1000000
#define v2d(rowsize,colsize,type,name) vector<vector<type>> name(rowsize,vector<type>(colsize)); 
#define digits_in(i) (ll)log10(i)+1 // gives no of digits in a number 
//does not apply for i==0 , add an excetion contition for n==0 ( cust return count 1 for that inseted of using this function)


//typedef
typedef string str;
typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<str> vs;
typedef vector<char> vc;
typedef pair<int,int> pii;
typedef pair<str,int> psi;
typedef pair<int,str> pis; 
typedef vector<pii> vii;
typedef map<int,int> mii;
typedef map<ll,ll> mll;
typedef map<str,int> msi;
typedef map<char,int> mci;
typedef map<int,str> mis;
typedef unordered_map<int,int> umii;
typedef unordered_map<str,int> umsi;
typedef unordered_map<int,str> umis;
typedef unordered_map<str,str> umss;
typedef unordered_map<char,int> umci;
typedef set<str> ss;
typedef set<int> si;
typedef unordered_set<str> uss;
typedef unordered_set<int> usi;
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> pbds;


#define DEBUG 

#ifdef DEBUG

    #define debug(args...)	cout << "Line No = " <<  __LINE__ << " >\t"; (Debugger()) , args

class Debugger
{
public:
	Debugger(const std::string& _separator = ", ") :
	first(true), separator(_separator){}

        template<typename ObjectType>
	Debugger& operator , (const ObjectType& v)
	{
		if(!first)
			std::cout << separator;
		std::cout << v;
		first = false;
		return *this;
	}
	~Debugger() {  std::cout << endl;}

private:
	bool first;
	std::string separator;
};

    template <typename T1, typename T2>
inline std::ostream& operator << (std::ostream& os, const std::pair<T1, T2>& p)
{
	return os << "(" << p.first << ", " << p.second << ")";
}

    template<typename T>
inline std::ostream &operator << (std::ostream & os,const std::vector<T>& v)
{
	bool first = true;
	os << "[";
	for(unsigned int i = 0; i < v.size(); i++)
	{
		if(!first)
			os << ", ";
		os << v[i];
		first = false;
	}
	return os << "]";
}

    template<typename T>
inline std::ostream &operator << (std::ostream & os,const std::set<T>& v)
{
	bool first = true;
	os << "[";
	for (typename std::set<T>::const_iterator ii = v.begin(); ii != v.end(); ++ii)
	{
		if(!first)
			os << ", ";
		os << *ii;
		first = false;
	}
	return os << "]";
}

    template<typename T1, typename T2>
inline std::ostream &operator << (std::ostream & os,const std::map<T1, T2>& v)
{
	bool first = true;
	os << "[";
	for (typename std::map<T1, T2>::const_iterator ii = v.begin(); ii != v.end(); ++ii)
	{
		if(!first)
			os << ", ";
		os << *ii ;
		first = false;
	}
	return os << "]";
}
template<typename T>
inline std::ostream &operator << (std::ostream & os,const std::deque<T>& v)
{
	bool first = true;
	os << "[";
	for (auto s: v)
	{
		if(!first)
			os << ", ";
		os << s;
		first = false;
	}
	return os << "]";
}
template<typename T>
inline std::ostream &operator << (std::ostream & os,const std::list<T>& v)
{
	bool first = true;
	os << "[";
	for (auto s: v)
	{
		if(!first)
			os << ", ";
		os << s;
		first = false;
	}
	return os << "]";
}

#else
    #define debug(args...)                  // Just strip off all debug tokens
#endif




/*   FILE IO 
     freopen("in.txt","r",stdin);    //to be put inside
     freopen("out.txt","w",stdout);  //main , when  used today
*/  




     int valid(vi & v){
     	re1(i,sz(v)-1)if(v[i]==i)return 0;
     	return 1;
     }

     int main(){fastio

     	int n;cin>>n;
     	vi v(n+1);

     	re1(i,n)cin>>v[i];
     	int ct=0;
     	int index=1;

     	while(!valid(v)){

     		while(v[index]!=index){
     			index++;
     			if(index>n)index=1;
     		}


     		if(index==n)swap(v[index],v[index-1]);
     		else swap(v[index],v[index+1]);
     		ct++;
     		

     	}

     	cout<<ct;
     	return 0;
     }








Measured memory_bytes: 640