#include<bits/stdc++.h>
typedef long long ll;
#define pb push_back
#define fr first
#define sc second
using namespace std;
int n;
int arr[200023];
ll po[200023],ne[200023];
int main(){
ios_base::sync_with_stdio(false);cin.tie(NULL);
cin>>n;
for(int i=1;i<=n;i++){
cin>>arr[i];
po[i]=po[i-1];
if(arr[i-1]-arr[i]>=0){
po[i]+=arr[i-1]-arr[i]+1;
}
}
for(int i=n;i>1;i--){
ne[i]=ne[i+1];
if(arr[i-1]-arr[i]<=0){
ne[i]-=arr[i-1]-arr[i]-1;
}
}
ll ans=123123123123123123;
for(int i=1;i<=n;i++){
ans=min(ans,max(po[i],ne[i+1]));
}
cout<<ans;
}
