#include <bits/stdc++.h>
#define se second
#define fs first
#define mp make_pair
#define pb push_back
#define ll long long
#define ii pair<ll,ll>
#define ld long double
#define SZ(v) (int)v.size()
#define ALL(v) v.begin(), v.end()
#define bit(msk, i) ((msk >> i) & 1)
#define iter(id, v) for(auto id : v)
#define rep(i,m,n) for(int i=(m); i<=(n); i++)
#define reb(i,m,n) for(int i=(m); i>=(n); i--)
using namespace std;
mt19937_64 rd(chrono :: steady_clock :: now().time_since_epoch().count());
ll Rand(ll l, ll r) { return uniform_int_distribution<ll> (l, r)(rd); }
const int N = 2e5 + 7;
const int Mod = 1e9 + 7;
const int szBL = 320;
const ll INF = 1e18;
const int BASE = 137;
int n, Q;
ll a[N];
ll pret[N], suft[N];
void solution () {
cin >> n;
rep (i, 1, n) cin >> a[i];
rep (i, 1, n) {
pret[i] = pret[i - 1] + max(0LL, a[i] + 1- a[i + 1]);
}
reb (i, n, 1) {
suft[i] = suft[i + 1] + max(0LL, a[i] + 1 - a[i - 1]);
}
ll res = INF;
rep (i, 0, n) {
res = min(res, pret[i] + max(0LL, suft[i + 2] - pret[i]));
}
cout << res <<"\n";
}
#define file(name) freopen(name".inp", "r", stdin); \
freopen(name".out", "w", stdout);
int main () {
// file("c");
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
ll num_Test = 1;
// cin >> num_Test;
while(num_Test--)
solution();
}
