#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pi;
typedef pair<ll,ll> pl;
#define eb emplace_back
#define all(v) v.begin(),v.end()
#define ff first
#define ss second
const ll mxn=2e5+5;
ll a[mxn],l[mxn],r[mxn];
int main()
{
ios::sync_with_stdio(0); cin.tie(0);
ll n; cin>>n;
for (int i=1;i<=n;i++) cin>>a[i];
for (int i=1;i<=n;i++) l[i]=l[i-1]+max(a[i-1]-a[i]+1,0ll);
for (int i=n;i>=1;i--) r[i]=r[i+1]+max(a[i+1]-a[i]+1,0ll);
ll rt=LLONG_MAX;
for (int i=1;i<=n;i++) rt=min(rt,max(l[i],r[i]));
cout<<rt;
}
