// TEMPLATE
#include <bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
#define ordered_set tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>
#define ordered_set_char tree<char, null_type, less<char>, rb_tree_tag, tree_order_statistics_node_update>
#define ordered_set_string tree<string, null_type, less<string>, rb_tree_tag, tree_order_statistics_node_update>
#define ordered_multiset tree<int, null_type, less_equal<int>, rb_tree_tag, tree_order_statistics_node_update>
#define ordered_multiset_char tree<char, null_type, less_equal<char>, rb_tree_tag, tree_order_statistics_node_update>
#define ordered_multiset_string tree<string, null_type, less_equal<string>, rb_tree_tag, tree_order_statistics_node_update>
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
#define int long long
#define ldouble long double
#define ll unsigned long long
// #define int int_fast32_t
#define endl '\n'
#define pii pair<int, int>
#define F first
#define S second
void solve()
{
int n;
cin >> n;
vector<int> vt(n);
for (int i = 0; i < n; i++)
{
cin >> vt[i];
}
vector<int> pref(n);
pref[0] = 0;
for (int i = 1; i < n; i++)
{
if (vt[i] <= vt[i - 1])
{
pref[i] = pref[i - 1] + vt[i - 1] + 1 - vt[i];
}
else
{
pref[i] = pref[i - 1];
}
}
vector<int> suf(n);
suf[n - 1] = 0;
for (int i = n - 2; i >= 0; i--)
{
if (vt[i] <= vt[i + 1])
{
suf[i] = suf[i + 1] + vt[i + 1] + 1 - vt[i];
}
else
{
suf[i] = suf[i + 1];
}
}
int ans = LLONG_MAX;
for (int i = 0; i < n; i++)
{
ans = min(ans, max(pref[i], suf[i]));
}
cout << ans << endl;
}
signed main()
{
ios::sync_with_stdio(false);
cin.tie(0);
int T = 1;
// cin >> T;
while (T--)
solve();
}
