#include<bits/stdc++.h>
using namespace std;
#define int long long
#define ff first
#define ss second
#define pb push_back
const int N=3e5+5;
int a[N];
signed main(){
int n;
cin>>n;
for(int i=0;i<n;i++){
cin>>a[i];
}
vector<int>v(n);
for(int i=0;i<n;i++)v[i]=i;
int ans=n*n;
do{
vector<int>tmp(n);
for(int i=0;i<n;i++){
tmp[i]=a[v[i]];
}
bool u=1;
int k=0;
for(int i=0;i<n;i++){
for(int j=i+1;j<n;j++){
if(tmp[j]<tmp[i] and tmp[i]-tmp[j] >= (j-i)*2){
u=0;
}
if(v[j]<v[i])k++;
}
}
if(u)ans=min(ans,k);
}while(next_permutation(v.begin(),v.end()));
cout<<ans;
}
