#include <bits/stdc++.h>
using namespace std;
#define OPTM ios_base::sync_with_stdio(0); cin.tie(0);
#define INF int(1e9+7)
#define ln '\n'
#define ll long long
#define ull unsigned long long
#define ui unsigned int
#define us unsigned short
#define FOR(i,s,n) for (int i = s; i < n; i++)
#define FORR(i,n,s) for (int i = n; i > s; i--)
#define FORX(u, arr) for (auto u : arr)
#define PB push_back
#define in(v,x) (v.find(x) != v.end())
#define F first
#define S second
#define PII pair<int, int>
#define PLL pair<ll, ll>
#define UM unordered_map
#define US unordered_set
#define PQ priority_queue
#define ALL(v) v.begin(), v.end()
const ll LLINF = 1e18+1;
#define int long long
const int MAXN = 2e5+1;
int n;
int a[MAXN], pl[MAXN], b[MAXN];
signed main() {
OPTM;
cin >> n;
FOR(i,1,n+1) cin >> a[i];
int c = a[1];
FOR(i,2,n+1) {
c = max(a[i], c+1);
pl[i] = c-a[i];
}
c = a[n];
FORR(i,n-1,0) {
c = max(a[i], c+1);
b[i] = c-a[i];
}
int cur = 0;
FOR(i,1,n+1) cur += max(0ll, b[i]-b[i-1]);
int res = cur;
FOR(i,2,n+1) {
int old = max(0ll,b[i-1]-b[i-2])+max(0ll,b[i]-b[i-1]);
if (i < n) old += max(0ll,b[i+1]-b[i]);
b[i-1] = pl[i-1];
b[i] = max(b[i-1], max(b[i], pl[i]));
int ne = max(0ll,b[i-1]-b[i-2])+max(0ll,b[i]-b[i-1]);
if (i < n) ne += max(0ll,b[i+1]-b[i]);
cur += ne-old;
res = min(res, cur);
}
cout << res;
}
