#include<bits/stdc++.h>
using namespace std;
#define ll long long
ll n,a[100005],l[100005],r[100005],hl[100005],hr[100005],ans;
int main()
{
ios_base::sync_with_stdio(0),cin.tie(0);
cin >> n;
for(int i=1;i<=n;i++)cin >> a[i];
l[1]=0,hl[1]=a[1];
for(int i=2;i<=n;i++)
{
if(a[i]>hl[i-1])
{
hl[i]=a[i];
l[i]=l[i-1];
}else
{
hl[i]=hl[i-1]+1;
l[i]=l[i-1]+hl[i]-a[i];
}
}
r[n]=0,hr[n]=a[n];
for(int i=n-1;i>=1;i--)
{
if(a[i]>hr[i+1])
{
hr[i]=a[i];
r[i]=r[i+1];
}else
{
hr[i]=hr[i+1]+1;
r[i]=r[i+1]+hr[i]-a[i];
}
}
/*for(int i=1;i<=n;i++)
{
printf("%d ",l[i]);
}
printf("\n");
for(int i=1;i<=n;i++)
{
printf("%d ",r[i]);
}
printf("\n");*/
ans=min(hl[n]-hl[1],hr[1]-hr[n]);
for(int i=1;i<n;i++)ans=min(ans,hl[i]+hr[i+1]-hl[1]-hr[n]);
cout << ans;
}
