#include <bits/stdc++.h>
// #include <ext/rope>
// #include <ext/pb_ds/assoc_container.hpp>
// using namespace __gnu_pbds;
// using namespace __gnu_cxx;
using namespace std;
// #define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
#define ll long long
#define ull unsigned long long
#define ld long double
#define pb push_back
#define bit(mask, i) ((mask >> i) & 1)
#define el '\n'
#define F first
#define S second
template <class X, class Y> bool maximize(X &x, const Y &y) { return (x < y ? x = y, 1 : 0); }
template <class X, class Y> bool minimize(X &x, const Y &y) { return (x > y ? x = y, 1 : 0); }
const int INF = 1e9;
const ll LINF = 1e18;
const int MOD = 1e9 + 7;
const int MULTI = 0;
const ld eps = 1e-9;
const int dx[4] = {0, 1, 0, -1}, dy[4] = {1, 0, -1, 0}; // R D L U
const int ddx[4] = {-1, 1, 1, -1}, ddy[4] = {1, 1, -1, -1}; // UR DR DL UL
const char cx[4] = {'R', 'D', 'L', 'U'};
const ll base = 31;
const int nMOD = 2;
const ll mods[] = {(ll)1e9 + 10777, (ll)1e9 + 19777, (ll)1e9 + 3, (ll)1e9 + 3777};
const int N = 2e5 + 5;
int n, a[N];
ll pref[N], suf[N], sufDiff[N];
void solve() {
cin >> n;
for (int i = 1; i <= n; ++i)
cin >> a[i];
int lst = 0;
for (int i = 1; i <= n; ++i) {
pref[i] = pref[i - 1];
if (a[i] > lst)
lst = a[i];
else {
lst++;
pref[i] += lst - a[i];
}
}
lst = 0;
for (int i = n; i >= 1; --i) {
suf[i] = suf[i + 1];
if (a[i] > lst)
lst = a[i];
else {
lst++;
suf[i] += lst - a[i];
}
}
for (int i = n - 1; i >= 1; --i)
sufDiff[i] = sufDiff[i + 1] + max(0, a[i + 1] - a[i]);
ll res = min(pref[n], suf[1]);
for (int i = 2; i < n; ++i) {
ll cur = pref[i - 1];
int h = max(pref[i] - pref[i - 1], suf[i] - suf[i + 1]);
int pr = pref[i - 1] - pref[i - 2];
cur += max(0, h - pr);
pr = suf[i + 1] - suf[i + 2];
cur += max(0, pr - h);
cur += sufDiff[i + 1];
minimize(res, cur);
}
cout << res;
}
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
if (!MULTI) solve();
else {
int test; cin >> test;
while (test--) solve();
}
return 0;
}
