#include <bits/stdc++.h>
#define For(i, a, b) for(long long i = a; i < b; i++)
#define rep(i, n) For(i, 0, n)
#define rFor(i, a, b) for(long long i = a; i >= b; i--)
#define ALL(v) (v).begin(), (v).end()
#define rALL(v) (v).rbegin(), (v).rend()
using namespace std;

using lint = long long;
using ld = long double;

int main() {
    int n;
    cin >> n;
    vector<int> a(n);
    rep(i, n) {
        cin >> a[i];
    }
    int sum = accumulate(ALL(a), 0);
    int ans = 2000000000;
    for (int i = 1; ; i += 2) {
        int h = (i + 1) / 2;
        if (h * h > sum) {
            break;
        }
        vector<int> len(i);
        for (int j = 0; j < h; j++) {
            len[j] = len[i - 1 - j] = j + 1;
        }
        int more = 0, less = 0;
        rep(j, max(i, n)) {
            if (j < n && j < i) {
                if (a[j] > len[j]) {
                    more += a[j] - len[j];
                } else {
                    less += len[j] - a[j];
                }
            } else if (j < n) {
                more += a[j];
            } else {
                less += len[j];
            }
        }
        ans = min(ans, max(more, less));
    }
    cout << ans << endl;
}