#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
int n;
cin >> n;
vector<ll> a(n);
for (auto& x : a) cin >> x;
if (n == 2) {
cout << (a[1] == a[0]) << "\n";
return 0;
}
ll res = LLONG_MAX;
for (int i = 0; i < n; ++i) {
ll now = 0;
ll ps = 0, sf = 0;
auto na = a;
for (int j = 0; j < i; ++j) {
now += max(0LL, ps - a[j] + 1);
na[j] += max(0LL, ps - a[j] + 1);
ps = max(ps + 1, na[j]);
}
for (int j = n - 1; j > i; --j) {
now += max(0LL, sf - a[j] + 1);
na[j] += max(0LL, sf - a[j] + 1);
sf = max(sf + 1, na[j]);
}
now += max(0LL, max(ps, sf) - a[i] + 1);
na[i] += max(0LL, max(ps, sf) - a[i] + 1);
//~ for (auto& x : na) {
//~ cout << x << " ";
//~ }
now = 0;
ll add = 0;
for (int j = 0; j <= n; ++j) {
if (j == n) {
now += add;
continue;
}
assert(a[j] <= na[j]);
if (a[j] != na[j] || j == i) {
add = max(add, na[j] - a[j]);
} else {
now += add;
add = 0;
}
}
//~ cout << " = " << now << "\n";
res = min(res, now);
}
cout << res << "\n";
}
