#include<bits/stdc++.h>
#define IOS ios_base::sync_with_stdio(false);cin.tie();cout.tie();
#define all(x) x.begin(), x.end()
#define int long long
#define pq priority_queue
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
#define pb push_back
#define pp pop_back
#define F first
#define S second
using namespace std;
int n;
int a[200010];
int h1[200010];
int h2[200010];
int ans = 1e18;
void solve () {
cin >> n;
for (int i = 1; i <= n; i++) cin >> a[i];
for (int i = 1; i <= n + 1; i++) h1[i] = h1[i-1] + max(0LL, a[i-1] - a[i] + 1);
for (int i = n; i >= 1 - 1; i--) h2[i] = h2[i+1] + max(0LL, a[i+1] - a[i] + 1);
for (int i = 1; i <= n; i++) ans = min(ans, max(h1[i], h2[i]));
cout << ans;
}
signed main() {IOS solve(); return 0;}
