001

#include <bits/stdc++.h>
using namespace std;
int n,i,maxb,maxc,minb,minc,b[1000005],c[1000005],a[1000005];
int main(){
    cin>>n;
    maxb=-INT_MAX;
    maxc=-INT_MAX;
    minb=INT_MAX;
    minc=INT_MAX;
    for (i=1;i<=n;i++) cin>>a[i];
    for (i=1;i<=n;i++) {b[i]=a[i]+i; {if (b[i]>maxb) maxb=b[i]; if (b[i]<minb) minb=b[i];} }
    for (i=1;i<=n;i++) {c[i]=a[i]-i; {if (c[i]>maxc) maxc=c[i]; if (c[i]<minc) minc=c[i];} }
    cout<<max(maxb-minb,maxc-minc);
    return 0;
}