#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define REP(i, n)           for(int(i)=0;(i)<(n);++(i))
#define REPEAT(i, k, n)     for(int(i)=(k);(i)<((k)+(n));++(i))

int k[1000];
int l[1000];
int m[1000];

int main(){
    int N;
    cin >> N;
    REP(i,N) cin >> k[i];
    
    int minv = 1<<29;
    for(int i = 1; i < 1000; i += 2){
        memcpy(l, k, sizeof(k));
        memset(m,0,sizeof(m));

        for(int j = 0; j < i; j++){
            m[j] = j+1;
            if(j > i/2) m[j] = i-j;
        }

        int c = 0;
        for(int j = 0; j < max(i,N); j++){
            if(l[j] <= m[j]) continue;
            int n = l[j]-m[j]; l[j] = m[j];
            c += n;
            for(int k = 0; k < N && n; k++){
                while(n && l[k] < m[k]){
                    l[k]++, n--;
                }
            }
        }
        bool f = true;
        for(int j = 0; j < i; j++){
            if(l[j] != m[j]) {f = false; break; }
        }
        if(f) minv = min(minv, c);
    }
    cout << minv << endl;

    return 0;
}