/******************************************************************************
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*******************************************************************************/
#include <bits/stdc++.h>
using namespace std;
#define int long long
signed main()
{
int n;
cin>>n;
int arr[n],a[n];
for(int i=0;i<n;i++){
cin>>arr[i];
a[i]=arr[i];
}
int pre[n+1]={},suf[n+1]={},ansP[n+1]={},ansS[n+1]={};
for(int i=1;i<n;i++){
if(pre[i]<=arr[i-1]-arr[i]){
pre[i]=((arr[i-1]-arr[i]))+1;
arr[i]=arr[i-1]+1;
}
}
for(int i=n-2;i>=0;i--){
if(suf[i]<=a[i+1]-a[i]){
suf[i]=(a[i+1]-a[i])+1;
a[i]=a[i+1]+1;
}
}
int cnt=0;
for(int i=1;i<n;i++){
if(pre[i]-pre[i-1]>0) cnt+=pre[i]-pre[i-1];
ansP[i]=cnt;
}
cnt=0;
for(int i=n-2;i>=0;i--){
if(suf[i]-suf[i+1]>0) cnt+=suf[i]-suf[i+1];
ansS[i]=cnt;
}
int mn=1e18;
for(int i=0;i<n;i++){
if(i==0) mn=min(mn,ansS[i]);
else if(i==n-1) mn=min(mn,ansP[i]);
else {
int ANS=max(ansP[i],ansS[i+1]);
if(a[i+1]==arr[i]) ANS++;
mn=min(ANS,mn);
}
}
cout<<mn<<endl;
return 0;
}
