#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define endl '\n'
#define int long long
const int N=2e5+5,inf=2e9,MOD=1e9+7;
int a[N],pref[N],suff[N];
signed main(){
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int n; cin>>n;
for(int i=1;i<=n;i++){
cin>>a[i];
pref[i]=pref[i-1];
if(i>1 && a[i]<=a[i-1])pref[i]+=a[i-1]-a[i]+1;
}
int sol=inf;
for(int i=n;i>0;i--){
suff[i]=suff[i+1];
if(i<n && a[i+1]>=a[i])suff[i]+=a[i+1]-a[i]+1;
sol=min(sol,max(pref[i],suff[i]));
//cout<<pref[i]<<' '<<suff[i]<<' '<<i<<endl;
}
cout<<sol;
}
