#include <bits/stdc++.h>
#define For(i,a,b) for(int i=a;i<=b;i++)
#define Forr(i,a,b) for(int i=a;i>=b;i--)
#define F first
#define S second
#define all(x) x.begin(),x.end()
#define sz(x) ((int)x.size())
#define eb emplace_back
#define INF (1000000000000000ll);
#define int ll
using namespace std;
using ll=long long;
using pii=pair<int,int>;
int a[200020];
int l[200020];
int r[200020];
int32_t main(){
ios::sync_with_stdio(false);
cin.tie(0);
// nachoneko sama & miku sama bless me >/////<
int n; cin>>n;
For(i,1,n) cin>>a[i];
For(i,2,n) l[i]=l[i-1]+max(0ll,a[i-1]-a[i]+1);
Forr(i,n-1,1) r[i]=r[i+1]+max(0ll,a[i+1]-a[i]+1);
int ans=INF;
For(i,1,n) ans=min(ans,max(l[i],r[i]));
cout<<ans<<"\n";
return 0;
}
