#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 100000000

vector<int> get(int n){
	vector<int> ret;
	for(int i=1;i<=n;i++)ret.push_back(i);
	for(int i=n-1;i>=1;i--)ret.push_back(i);
	return ret;
}

int main(){
	int N;
	cin>>N;
	vector<int> A(N);
	int as = 0;
	rep(i,N){
		cin>>A[i];
		as += A[i];
	}
	
	int ans = Inf;
	
	for(int i=1;i<=10000000;i++){
		auto t = get(i);
		int s = 0;
		rep(j,t.size())s += t[j];;
		if(s > as)break;
		while(t.size()<N)t.push_back(0);
		s = 0;
		rep(j,A.size()){
			s += max(0,A[j] - t[j]);
		}
		ans = min(ans,s);
		
		
	}
	cout<<ans<<endl;
	
	return 0;
}