#include <iostream>
#include <algorithm>
#include <assert.h>
#include <string>
using namespace std;

long long readInt(long long l,long long r,char endd){
	long long x=0;
	int cnt=0;
	int fi=-1;
	bool is_neg=false;
	while(true){
		char g=getchar();
		if(g=='-'){
			assert(fi==-1);
			is_neg=true;
			continue;
		}
		if('0'<=g && g<='9'){
			x*=10;
			x+=g-'0';
			if(cnt==0){
				fi=g-'0';
			}
			cnt++;
			assert(fi!=0 || cnt==1);
			assert(fi!=0 || is_neg==false);
			
			assert(!(cnt>19 || ( cnt==19 && fi>1) ));
		} else if(g==endd){
			assert(cnt>0);
			if(is_neg){
				x= -x;
			}
			assert(l<=x && x<=r);
			return x;
		} else {
			assert(false);
		}
	}
}
string readString(int l,int r,char endd){
	string ret="";
	int cnt=0;
	while(true){
		char g=getchar();
		assert(g!=-1);
		if(g==endd){
			break;
		}
		cnt++;
		ret+=g;
	}
	assert(l<=cnt && cnt<=r);
	return ret;
}
long long readIntSp(long long l,long long r){
	return readInt(l,r,' ');
}
long long readIntLn(long long l,long long r){
	return readInt(l,r,'\n');
}
string readStringLn(int l,int r){
	return readString(l,r,'\n');
}
string readStringSp(int l,int r){
	return readString(l,r,' ');
}

int T;
int n;
int arr[100100];
int dp[100100][2];

bool compare(int a,int b,bool n){
	if(n){
		return a<b;
	}
	return a>b;
}
int f(int i){
	return n-i+1;
}

int solve_even(int is_bigger){
	for(int i=1;i<=n;i++){
		dp[i][0]=dp[i][1]=0;
	}
	dp[n/2 + 1][0] = 1<<20;
	dp[n/2 + 1][1] = 1<<20;
	if(!is_bigger){
		if(arr[n/2 ] < arr[n/2 + 1]){
			dp[n/2 + 1][0] = 0;
		}
		if(arr[n/2 ] > arr[n/2 + 1]){
			dp[n/2 + 1][1] = 1;
		}
	} else {
		if(arr[n/2 ] > arr[n/2 + 1]){
			dp[n/2 + 1][0] = 0;
		}
		if(arr[n/2 ] < arr[n/2 + 1]){
			dp[n/2 + 1][1] = 1;
		}
	}
	for(int i=n/2+2;i<=n;i++){
		dp[i][0]= 1<<20;
		dp[i][1]= 1<<20;
		if(compare(arr[i-1],arr[i],is_bigger) && compare(arr[f(i)],arr[f(i-1)],is_bigger)){
			dp[i][0]=min(dp[i][0],dp[i-1][0]);
		} 
		if(compare(arr[f(i-1)],arr[i],is_bigger) && compare(arr[f(i)],arr[i-1],is_bigger)){
			dp[i][0]=min(dp[i][0],dp[i-1][1]);
		} 
		if(compare(arr[i-1],arr[f(i)],is_bigger) && compare(arr[i],arr[f(i-1)],is_bigger)){
			dp[i][1]=min(dp[i][1],dp[i-1][0]+1);
		} 
		if(compare(arr[f(i-1)],arr[f(i)],is_bigger) && compare(arr[i],arr[i-1],is_bigger)){
			dp[i][1]=min(dp[i][1],dp[i-1][1]+1);
		} 
		is_bigger = !is_bigger;
	}
	int ans =min(dp[n][0],dp[n][1]);
	return ans;
}


int solve_odd(bool is_bigger){
	for(int i=1;i<=n;i++){
		dp[i][0]=dp[i][1]=0;
	}
	dp[n/2+1][1]=1<<20;
	for(int i=n/2+2;i<=n;i++){
		dp[i][0]= 1<<20;
		dp[i][1]= 1<<20;
		if(compare(arr[i-1],arr[i],!is_bigger) && compare(arr[f(i)],arr[f(i-1)],is_bigger)){
			dp[i][0]=min(dp[i][0],dp[i-1][0]);
		} 
		if(compare(arr[f(i-1)],arr[i],!is_bigger) && compare(arr[f(i)],arr[i-1],is_bigger)){
			dp[i][0]=min(dp[i][0],dp[i-1][1]);
		} 
		if(compare(arr[i-1],arr[f(i)],!is_bigger) && compare(arr[i],arr[f(i-1)],is_bigger)){
			dp[i][1]=min(dp[i][1],dp[i-1][0]+1);
		} 
		if(compare(arr[f(i-1)],arr[f(i)],!is_bigger) && compare(arr[i],arr[i-1],is_bigger)){
			dp[i][1]=min(dp[i][1],dp[i-1][1]+1);
		} 
		is_bigger = !is_bigger;
	}
	int ans =min(dp[n][0],dp[n][1]);
	return ans;
}
int main(){
	//freopen("in0.txt","r",stdin);
	//n=readIntLn(1,100000);
	cin.sync_with_stdio(false);
	cin>>n;
	for(int i=1;i<=n;i++){
		if(i==n){
			//arr[i] = readIntLn(1,1000000000);
		} else {
			//arr[i] = readIntSp(1,1000000000);
		}
		cin>>arr[i];
	}
	

	int ans;
	if(n % 2==0){
		ans = solve_even(true);
		
		ans = min(ans,solve_even(false));
	} else {
		ans = solve_odd(true);
		ans = min(ans,solve_odd(false));
	}
	if(ans == 1<<20){
		cout<<-1<<endl;
	} else {
		cout<<ans<<endl;
	}
	//assert(getchar()==-1);
}