#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
typedef long long ll;
typedef pair<int,int> ii;
const int N = 2e5;
const ll INF = 1e18;
int n;
int a[N+2], b[N+2];
int main() {
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
cin >> n;
for (int i = 1; i <= n; i++) {
cin >> a[i];
}
ll ans = INF;
for (int i = 1; i <= n; i++) {
for (int j = 1; j <= i-1; j++) b[j] = max(b[j-1]+1,a[j]);
for (int j = n; j >= i+1; j--) b[j] = max(b[j+1]+1,a[j]);
b[i] = max(max(b[i-1],b[i+1])+1,a[i]);
ll cost = 0;
for (int j = 1; j <= n; j++) {
b[j] -= a[j];
cost += max(0,b[j]-b[j-1]);
}
ans = min(ans,cost);
}
cout << ans << '\n';
return 0;
}
