/*
* Author: Nonoze
* Created: Saturday 20/12/2025
*/
#include <bits/stdc++.h>
using namespace std;
#ifndef DEBUG
#define dbg(...)
#endif
// #define cout cerr << "OUT: "
#define endl '\n'
#define endlfl '\n' << flush
#define quit(x) return (void)(cout << x << endl)
template<typename T> void read(T& x) { cin >> x; }
template<typename T1, typename T2> void read(pair<T1, T2>& p) { read(p.first), read(p.second); }
template<typename T> void read(vector<T>& v) { for (auto& x : v) read(x); }
template<typename T1, typename T2> void read(T1& x, T2& y) { read(x), read(y); }
template<typename T1, typename T2, typename T3> void read(T1& x, T2& y, T3& z) { read(x), read(y), read(z); }
template<typename T1, typename T2, typename T3, typename T4> void read(T1& x, T2& y, T3& z, T4& zz) { read(x), read(y), read(z), read(zz); }
template<typename T> void print(vector<T>& v) { for (auto& x : v) cout << x << ' '; cout << endl; }
#define sz(x) (int)(x.size())
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define make_unique(v) sort(all(v)), v.erase(unique(all(v)), (v).end())
#define pb push_back
#define mp(a, b) make_pair(a, b)
#define fi first
#define se second
#define cmin(a, b) a = min(a, b)
#define cmax(a, b) a = max(a, b)
#define YES cout << "YES" << endl
#define NO cout << "NO" << endl
#define QYES quit("YES")
#define QNO quit("NO")
#define int long long
#define double long double
const int inf = numeric_limits<int>::max() / 4;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
const int MOD = 1e9+7, LOG=20;
void solve();
signed main() {
ios::sync_with_stdio(0);
cin.tie(0);
int tt=1;
// cin >> tt;
while(tt--) solve();
return 0;
}
int n, k, m, q;
vector<int> a;
void solve() {
read(n);
a.clear(), a.resize(n); read(a);
vector<int> b=a;
for (int i=n-2; i>=0; i--) cmax(b[i], b[i+1]+1);
int cur=0;
for (int i=n-2; i>=0; i--) {
int addact=b[i]-a[i], addprec=b[i+1]-a[i+1];
if (addprec>addact) cur+=addprec-addact;
}
int ans=cur+max(0LL, b[0]-a[0]);
if (n>=2 && b[1]-a[1]>b[0]-a[0]) cur-=b[1]-a[1]- (b[0]-a[0]);
for (int i=1; i<n; i++) {
if (i+1<n) {
int addact=b[i]-a[i], addprec=b[i+1]-a[i+1];
if (addprec>addact) cur-=addprec-addact;
}
if (i==1) b[i-1]=a[i-1];
else b[i-1]=max(a[i-1], b[i-2]+1);
cmax(b[i], b[i-1]+1);
int right=0;
if (i+1<n) right=max(0LL, b[i+1]-a[i+1]);
int left=max(0LL, b[i-1]-a[i-1]);
// dbg(i, cur, left, right, b[i]-a[i], b);
cmin(ans, cur+max({0LL, left, right, b[i]-a[i]}));
b[i]=max(a[i], b[i-1]+1);
int addact=b[i]-a[i], addprec=b[i-1]-a[i-1];
if (addprec>addact) cur+=addprec-addact;
vector<int> c;
for (int j=0; j<n; j++) c.pb(b[j]-a[j]);
dbg(c);
}
cout << ans << endl;
}
