//#pragma GCC target("avx2")
//#pragma GCC optimization("O3")
//#pragma GCC optimization("unroll-loops")
#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define pii pair<int, int>
#define pll pair<ll, ll>
#define ff first
#define ss second
#define pb push_back
#define SZ(x) ((int)(x).size())
#define all(x) x.begin(), x.end()
#define rep(i, a, b) for(int i = (a); i < (b); i++)
#define Re(i, a, b) for(int i = (a); i >= (b); i--)
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
void solve(){
int N; cin >> N;
vector<int> a(N + 2);
rep(i, 1, N + 1) cin >> a[i];
vector<ll> l(N + 2, 0), r(N + 2, 0);
rep(i, 1, N + 1){
l[i] = l[i - 1] + max((a[i - 1] + 1) - a[i], 0);
}
Re(i, N, 1){
r[i] = r[i + 1] + max((a[i] + 1) - a[i - 1], 0);
}
ll ans = LLONG_MAX;
rep(i, 1, N + 1){
ans = min(ans, l[i] + max(r[i + 1] - l[i], (ll) 0));
}
cout << ans;
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
solve();
return 0;
}
