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

int main() {
    int N, A[110];
    cin >> N;
    for (int i = 0; i < N; i++) {
        cin >> A[i];
    }

    int SUM = 0;
    for (int i = 0; i < N; i++) {
        SUM += A[i];
    }

    int ans = 10000000;
    for (int i = 0; (i + 1) * (i + 1) <= SUM; i++) {
        int cost = 0;
        for (int j = 0; j < N; j++) {
            if (j <= i)
                cost += max(0, A[j] - j - 1);
            else if (j <= i * 2)
                cost += max(0, A[j] - (i * 2 + 1 - j));
            else
                cost += A[j];
        }
        ans = min(ans, cost);
    }

    cout << ans << endl;
}