#include <bits/stdc++.h>
using namespace std;
#define int long long
#define ld long double
#define pii pair<int, int>
#define fi first
#define se second
#define __builtin_popcount __builtin_popcountll
#define all(x) (x).begin(), (x).end()
#define BIT(x, i) (((x) >> (i)) & 1)
#define debug(a, l, r) for (int i = (l); i <= (r); ++i) cout << (a)[i] << ' '; cout << '\n';
const int MAXN = 2e5 + 5;
const int INF = 1e18 + 5;
int n, a[MAXN], pref[MAXN], suff[MAXN];
signed main() {
#ifdef NCTHANH
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
ios_base::sync_with_stdio(0);
cin.tie(nullptr); cout.tie(nullptr);
cin >> n;
for (int i = 1; i <= n; ++i) cin >> a[i];
{
pref[1] = 0;
for (int i = 2; i <= n; ++i) {
int cur = max(0ll, a[i - 1] + 1 - a[i]);
pref[i] = pref[i - 1] + cur;
}
}
{
suff[n] = 0;
for (int i = n - 1; i >= 1; --i) {
int cur = max(0ll, a[i + 1] + 1 - a[i]);
suff[i] = suff[i + 1] + cur;
}
}
int res = INF;
for (int i = 1; i <= n; ++i)
res = min(res, max(pref[i], suff[i]));
cout << res;
return 0;
}
