#include<iostream>

using namespace std;

#define NMAX 100

int main(){

	int N;
	int A[NMAX+1]={0};
	int sum[NMAX*2+1]={0};
	int suma=0;
	int ans=-1;
	int locd,loca;

	cin>>N;

	for(int i=1;i<=200;i+=2){
		int c=1;
		for(int j=0;j<i;j++){
			sum[i]+=c;
			if(i/2+1>j+1) c++;
			else c--;
		}
	}
	
	for(int i=1;i<=N;i++){
		cin>>A[i];
		suma+=A[i];
	}

	for(int i=1;;i+=2){
		if(sum[i]>suma) break;
		locd=0;
		loca=0;
		int c=1;
		for(int j=1;j<=i;j++){
			if(c>A[j]) loca+=c-A[j];
			else if(c<A[j]) locd+=A[j]-c;
			if(i/2+1>j) c++;
			else c--;
		}
		for(int j=i+1;j<=N;j++){
			locd+=A[j];
		}

		/*cout<<i<<","<<loca<<","<<locd<<endl;*/

		if(loca>locd) continue;
		if(ans==-1||locd<ans) ans=locd;
	}

	cout<<ans<<endl;

}