#include <bits/stdc++.h>
using namespace std;
#define ff first
#define ss second
#define pb push_back
#define int long long
int n,a[1000005],pref[1000005],suff[1000005];
int ans;
signed main(){
cin>>n;
for(int i=1;i<=n;i++){
cin>>a[i];
}
for(int i=2;i<=n;i++){
pref[i]=pref[i-1]+max(0LL,(1-(a[i]-a[i-1])));
}
for(int i=n-1;i>=1;i--){
suff[i]=suff[i+1]+max(0LL,((a[i+1]-a[i])+1));
}
ans=LONG_LONG_MAX;
for(int i=1;i<=n;i++){
// cout<<suff[i]<<" "<<pref[i]<<endl;
ans=min(ans,max(pref[i],suff[i]));
}
cout<<ans<<endl;
}
