#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define int ll
using P = pair<int, int>;
#define all(x) x.begin(), x.end()
#define rep(x,s,e) for (auto x=(s)-((s)>(e));x!=(e)-((s)>(e));((s)<(e)?x++:x--))
#define sz(x) (int)x.size()
const char nl = '\n';
const int mod = 998244353;
const int inf = 1e9*2e3+4e6;
vector<int> a, b;
int n;
void inc(int x) {
if (x)
rep(j, 1, x+1)
a[j] = max(a[j], a[j-1]+1);
}
void dec(int x) {
if(x!=n-1)
rep(j, n-1, x)
a[j] = max(a[j], a[j+1]+1);
}
int cost() {
vector<int> c = a;
rep(j, 0, n)c[j] -= b[j];
int sm = c[0];
rep(j, 1, n)sm += c[j]-min(c[j], c[j-1]);
return sm;
}
void solve() {
cin >> n;
a.resize(n);
for (auto &i: a)cin >> i;
b = a;
dec(0);
int res = cost(); a = b;
rep(i, 0, n) {
inc(i); dec(i);
res = min(res, cost()); a = b;
}
cout << res << nl;
}
int32_t main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
solve();
return 0;
}
