#include <bits/stdc++.h>
#define f first
#define s second
using namespace std;
#define int long long
signed main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int tt = 1;
//cin >> tt;
while(tt--) {
int n;
cin >> n;
vector<int> a(n);
for(int i = 0;i < n;i++)
cin >> a[i];
vector<int> p(n), s(n);
for(int i = 1;i < n;i++)
p[i] = p[i - 1] + max(0ll, a[i - 1] + 1 - a[i]);
for(int i = n - 2;i >= 0;i--)
s[i] = s[i + 1] + max(0ll, a[i + 1] + 1 - a[i]);
int ans = 1e18;
for(int i = 0;i < n;i++)
ans = min(ans, max(s[i], p[i]));
cout << ans;
}
return 0;
}
