#include<bits/stdc++.h>
using namespace std;
#define debug(x) cout << '[' << #x << " is: " << x << "] " << endl;
#define inF freopen("cardgame.in" , "r" , stdin);
#define outF freopen("cardgame.out" , "w" , stdout);
#define imod(a , n) (a % n + n ) % n
#define sor(v) sort(v.begin() , v.end());
#define print(v) for(auto f : v ) cout << f << " " ;
#define rsor(v) sort(v.rbegin() , v.rend());
#define rev(v) reverse(v.begin() , v.end());
#define scan(v)for(auto &it : v)cin >> it ;
#define ll long long
#define logar(x , y) log(x) / log(y)
#define __sum(n) n * (n + 1) / 2
#define __lcm(a , b) a / __gcd(a , b) * b
#define pii pair<int , int >
#define fastio ios_base::sync_with_stdio(false);cin.tie(0);
#define int ll
const int N = 2003, M = N * 4 , MX = 2e4 + 40;
const ll MOD = 998244353 , oo = 1e9 + 9 , OO = 1e18 , mod = MOD ;
const double pi = acos(-1) , eps = 1e-17 ;
int di[] = {1 , -1 , 0 , 0};
int dj[] = {0 , 0 , 1 , -1};
int32_t main()
{
// inF;
// inF;outF;
fastio;
//Welcome back without <TAWJIHI>
int n;
cin >> n ;
vector<ll> v(n);
for(auto &it : v)
cin >> it ;
ll mn = 1e18 ;
for(int i = 0 ;i < n ; i ++){
vector<ll> tmp = v;
for(int j = 1 ; j <= i ; j ++){
tmp[j] = max(tmp[j] , tmp[j - 1] + 1);
}
for(int j = n - 2 ; j >= i ; j --){
tmp[j] = max(tmp[j] , tmp[j + 1] + 1);
}
for(int j = 0 ; j < n ; j ++)tmp[j] = tmp[j] - v[j];
ll cnt = 0 , sum = 0;
for(int j = 0 ; j < n ; j ++){
if(tmp[j] <= sum)sum = tmp[j];
else {
cnt += tmp[j] - sum ;
sum = tmp[j];
}
}
mn = min(mn , cnt);
}
cout << mn ;
}
