#include<iostream>
#include<iomanip>
#include<string>
#include<algorithm>
#include<vector>
#include<set>
#include<list>
#include<queue>
#include<math.h>
#include<bitset>
using ll = long long;
using namespace std;

int main(){
    int n, sum1 = 0, ans = 10000;
    cin >> n;
    vector<int> a(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
        sum1 += a[i];
    }
    int maxn = pow(sum1, 0.5);
    for (int i = 1; i <= maxn; i++){
        int diff = 0;
        for (int j = 0; j < n; j++){
            if (j+1 <= i) {
                if (a[j] > (j+1)) diff += (a[j] - (j+1));
            } else if (j+1 < i*2) {
                if (a[j] > (2*i - (j+1))) diff += (a[j] - (2*i - (j+1)));
            } else diff += a[j];
        }
        ans = min(ans, diff);
    }
    cout << ans << endl;
}