//#pragma GCC optimize ("O2")
#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define fi first
#define se second
#define endl '\n'
#define task "task"
#define task "task"
#define prll pair<ll,ll>
#define pb push_back
#define ld long double
const ll MIN=-1e18,MAX=1e18,MOD=1e9+7,nmax=200005;
ll a[nmax],h1[nmax],h2[nmax],cost1[nmax],cost2[nmax];
int main(){
//freopen (task".inp", "r", stdin);
//freopen (task".out", "w", stdout);
ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
int n;
cin>>n;
for(int i=1;i<=n;i++)
{
cin>>a[i];
}
h1[1]=a[1];
for(int i=2;i<=n;i++)
{
if(a[i]<=a[i-1])
{
cost1[i]=cost1[i-1]+a[i-1]-a[i]+1;
h1[i]=h1[i-1]+1;
}
else
{
cost1[i]=cost1[i-1];
h1[i]=max(h1[i-1]+1,a[i]);
}
}
for(int i=n-1;i>=1;i--)
{
if(a[i]<=a[i+1])
{
cost2[i]=cost2[i+1]+a[i+1]-a[i]+1;
h2[i]=h2[i+1]+1;
}
else
{
cost2[i]=cost2[i+1];
h2[i]=max(h2[i+1]+1,a[i]);
}
}
// for(int i=1;i<=n;i++)
// {
// cout<<cost1[i]<<" ";
// }
// cout<<endl;
// for(int i=1;i<=n;i++)
// {
// cout<<h1[i]<<" ";
// }
// cout<<endl;
// for(int i=1;i<=n;i++)
// {
// cout<<cost2[i]<<" ";
// }
// cout<<endl;
// for(int i=1;i<=n;i++)
// {
// cout<<h2[i]<<" ";
// }
// cout<<endl;
// cout<<cost1[3]<<" "<<cost2[4];
ll ans=MAX;
for(int i=1;i<=n;i++)
{
ans=min(ans,max(cost1[i],cost2[i+1]));
}
cout<<ans;
}
/*
8
12 2 34 85 4 91 29 85
*/
