#include<bits/stdc++.h>
using namespace std;
const int MAXN = 2e5 + 5;
#define int long long
int n;
int a[MAXN];
int pre[MAXN];
int suf[MAXN];
int ans_pre[MAXN];
int ans_suf[MAXN];
signed main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin >> n;
for (int i = 1; i <= n; i++)
{
cin >> a[i];
}
for (int i = 2; i <= n; i++)
{
pre[i] = pre[i - 1] + max(0LL, a[i - 1] - a[i] + 1);
}
for (int i = n - 1; i >= 1; i--)
{
suf[i] = suf[i + 1] + max(0LL, a[i + 1] - a[i] + 1);
}
int ans = LLONG_MAX;
for (int i = 1; i <= n; i++)
{
ans = min(ans, max(pre[i], suf[i]));
}
cout << ans;
}
