#include<bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define pii pair<int,int>
#define pll pair<long long,long long>
#define pb push_back
#define debug(x) cerr<<#x<<"="<<x<<endl
#define pq priority_queue
#define inf (1ll<<60)
#define rep(i,a,b) for (int i=a;i<(b);i++)
#define MP make_pair
#define SZ(x) (int(x.size()))
#define ll long long
#define mod 1000000007
#define ALL(x) x.begin(),x.end()
#define endl "\n"
void inc(int &a,int b) {a=(a+b)%mod;}
void dec(int &a,int b) {a=(a-b+mod)%mod;}
int lowbit(int x) {return x&(-x);}
ll expo(ll base,ll p) {ll ret=1;while(p>0){if (p%2ll==1ll) ret=ret*base%mod;base=base*base%mod;p/=2ll;}return ret;}
const ll maxn=222222;
ll l[maxn],r[maxn],a[maxn];
int main(){
ios::sync_with_stdio(0);
cin.tie(0);
ll n;cin>>n;
rep(i,1,n+1)cin>>a[i];
rep(i,2,n+1){
l[i]=l[i-1]+max(0ll,l[i-1]-l[i]+1);
}
for(int i=n-1;i>=1;i--){
r[i]=r[i+1]+max(0ll,a[i+1]-a[i]+1);
}
//~ rep(i,1,n+1)cout<<l[i]<<" ";
//~ cout<<endl;
//~ rep(i,1,n+1)cout<<r[i]<<" ";
//~ cout<<endl;
ll ans=inf;
rep(i,1,n+1){
ans=min(ans,max(l[i],r[i]));
}
cout<<ans;
return 0;
}
/*
*/
