#include <cmath>
#include <deque>
#include <algorithm>
#include <iterator>
#include <list>
#include <tuple>
#include <map>
#include <unordered_map>
#include <queue>
#include <set>
#include <unordered_set>
#include <stack>
#include <string>
#include <vector>
#include <fstream>
#include <iostream>
#include <functional>
#include <numeric>
#include <iomanip> 
#include <stdio.h>
#include <assert.h>
//eolibraries
#define lnf 3999999999999999999
#define inf 999999999
#define fi first
#define se second
#define pb push_back
#define ll long long
#define ld long double
#define all(c) (c).begin(),(c).end()
#define sz(c) (int)(c).size()
#define make_unique(a) sort(all(a)),a.erase(unique(all(a)),a.end())
#define pii pair <int,int>
#define rep(i,n) for(int i = 0 ; i < n ; i++) 
#define drep(i,n) for(int i = n-1 ; i >= 0 ; i--)
#define crep(i,x,n) for(int i = x ; i < n ; i++)
#define vi vector <int> 
#define vec(...) vector<__VA_ARGS__>
#define fcin ios_base::sync_with_stdio(false),cin.tie(0),cout.tie(0);
//eodefine
using namespace std;

const int mxn=500000;

int main(){
fcin;
	int n;
	cin>>n;
	vi a(n);
	rep(i,n) {cin>>a[i];a[i]--;}
	int ans=0,i=0;
	vi pos(n,0);
	rep(_,n+2){
		bool pok=1;
		drep(i,n-1){
			if(a[i]>a[i+1]){
				pok=0;
				break;
			}
		}
		if(pok) break;
		rep(k,n) pos[a[k]]=k;
		while(i<n and a[i]==i) i++;
		if(a[i]!=i){
			int x=a[i];
			while(x!=i){
				if(a[i]>i){
					swap(a[pos[x-1]],a[i]);
					x--;
				}else{
					swap(a[pos[x+1]],a[i]);
					x++;
				}
				ans++;	
			}
		}
	}
	drep(i,n-1){
		if(a[i]>a[i+1]){
			cout<<"-1\n";
			exit(0);
		}
	}
	cout<<ans<<"\n";
/*
*/
	return 0;
}