#include <string>
#include <vector>
#include <algorithm>
#include <numeric>
#include <set>
#include <map>
#include <queue>
#include <iostream>
#include <sstream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstring>
#include <cctype>
#include <cassert>
#include <limits>
#include <functional>
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define rer(i,l,u) for(int (i)=(int)(l);(i)<=(int)(u);++(i))
#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
#if defined(_MSC_VER) || __cplusplus > 199711L
#define aut(r,v) auto r = (v)
#else
#define aut(r,v) __typeof(v) r = (v)
#endif
#define each(it,o) for(aut(it, (o).begin()); it != (o).end(); ++ it)
#define all(o) (o).begin(), (o).end()
#define pb(x) push_back(x)
#define mp(x,y) make_pair((x),(y))
#define mset(m,v) memset(m,v,sizeof(m))
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL
using namespace std;
typedef vector<int> vi; typedef pair<int,int> pii; typedef vector<pair<int,int> > vpii; typedef long long ll;
template<typename T, typename U> inline void amin(T &x, U y) { if(y < x) x = y; }
template<typename T, typename U> inline void amax(T &x, U y) { if(x < y) x = y; }

int main() {
	int N;
	while(~scanf("%d", &N)) {
		vector<int> A(N);
		for(int i = 0; i < N; ++ i)
			scanf("%d", &A[i]);
		int ans = INF;
		for(int n = 1; n <= 10000; n += 2) {
			int a = 0, b = 0, diff = 0;
			rep(i, max(N, n)) {
				int t = i < (n + 1) / 2 ? 1 + i : i < n ? n - i : 0;
				int u = i < N ? A[i] : 0;
				a += max(u - t, 0);
				b += max(t - u, 0);
				diff += u - t;
			}
			if(diff >= 0) {
				b += diff;
				amin(ans, a + b - min(a, b));
			}
		}
		printf("%d\n", ans);
	}
	return 0;
}