#include <bits/stdc++.h>
using namespace std;

int main(void) {
	int N;
	cin >> N;
	vector<int> A(N);
	for(int i = 0; i < N; ++i) cin >> A[i];
	
	int Asum = 0;
	for(int i = 0; i < N; ++i) Asum += A[i];
	
	int ans = 1e9;
	for(int t = 1; t * t <= Asum and t * 2 - 1 <= N; ++t) {
		vector<int> B(N, 0);
		for(int i = 0; i < t; ++i) B[i] = B[t * 2 - 2 - i] = i + 1;
		
		int rem = 0;
		for(int i = 0; i < N; ++i) if(A[i] > B[i]) rem += (A[i] - B[i]);
		
		ans = min(ans, rem);
	}
	
	cout << ans << "\n";
	
	return 0;
}