#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
using namespace __gnu_pbds;
using namespace std;
#define pb push_back
#define st first
#define nd second
typedef long long ll;
typedef long double ld;
const ll I = 1000'000'000'000'000'000LL;
const int II = 2'000'000'000;
const ll M = 1000'000'007LL;
const int N = 1000'007;
int tab[N], m1[N], m2[N];
ll s1[N], s2[N];
void Solve()
{
int n;
cin >> n;
for(int i = 1; i <= n; ++i)
cin >> tab[i];
for(int i = 1; i <= n; ++i)
{
m1[i] = max(tab[i], m1[i - 1] + 1);
s1[i] = s1[i - 1];
s1[i] += max(0, (m1[i] - tab[i]) - (m1[i - 1] - tab[i - 1]));
}
for(int i = n; i >= 1; --i)
{
m2[i] = max(tab[i], m2[i + 1] + 1);
s2[i] = s2[i + 1];
s2[i] += max(0, (m2[i] - tab[i]) - (m2[i + 1] - tab[i + 1]));
}
ll ans = I;
for(int i = 1; i <= n; ++i)
ans = min(ans, max(s1[i], s2[i]));
cout << ans << "\n";
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
//int t; cin >> t;
//while(t--)
Solve();
return 0;
}
