#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define all(x) (x).begin(), (x).end()
#define endl '\n'
class Solve {
public:
void solve() {
int n;
cin >> n;
vector<int> a(n);
for(int i = 0; i < n; i++) cin >> a[i];
vector<int> l(n);
l[0] = a[0];
for(int i = 1; i < n; i++) l[i] = max(a[i], l[i-1]+1);
vector<int> r(n);
r[n-1] = a[n-1];
for(int i = n-2; i >= 0; i--) r[i] = max(a[i], r[i+1]+1);
ll ans = 0;
for(int i = 0; i < n; i++) ans += max(l[i], r[i]) - a[i];
cout << ans << endl;
}
};
signed main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
Solve T;
T.solve();
}
