// ?
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> pll;
#define all(x) (x).begin(),(x).end()
#define X first
#define Y second
#define sep ' '
#define endl '\n'
#define debug(x) cerr << #x << ": " << x << endl;
const ll MAXN = 1e6 + 10;
const ll INF = 1e18;
ll A[MAXN], pref[MAXN], suff[MAXN], n;
int main() {
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
cin >> n;
for (int i = 1; i <= n; i++) {
cin >> A[i];
pref[i] = pref[i - 1] + max(0ll, A[i - 1] - A[i] + 1);
}
for (int i = n; i > 0; i--)
suff[i] = suff[i + 1] + max(0ll, A[i + 1] - A[i] + 1);
ll ans = INF;
for (int i = 1; i <= n; i++)
ans = min(ans, max(pref[i], suff[i]));
cout << ans << endl;
return 0;
}
