#include <bits/stdc++.h>
#define REP(i,n,N) for(int i=(n);i<(int) N;i++)
#define p(s) cout<<(s)<<endl
using namespace std;
const int inf=1e9;

int A[1010];
int B[1010];
int main(){
	int N;
	cin>>N;
	int sum=0;
	REP(i,0,N) {
		cin>>A[i];
		sum+=A[i];
	}

	int mn=inf;

	for(int k=1;(k+1)/2+(k+1)/2*((k+1)/2-1)<=sum;k+=2){
		int plus=0,minus=0;
		REP(i,0,(k+1)/2) B[i]=i+1;
		REP(i,(k+1)/2,k) B[i]=k-i;
		REP(i,k,N) B[i]=0;
		REP(i,0,N){
			if(A[i]>B[i]) plus+=A[i]-B[i];
			else if(A[i]<B[i]) minus+=B[i]-A[i];
		}
		mn=min(mn,max(plus,minus));
	}
	p(mn);
	return 0;
}