#include <bits/stdc++.h>
using namespace std;
#define int long long
signed main() {
int n;
cin>>n;
vector<int>a(n);
for(int i = 0;i<n;i++){
cin>>a[i];
}
vector<int>pref(n+1,0);
for(int i = 2;i<=n;i++){
if(a[i-1]<=a[i-2]+1){
pref[i] = pref[i-1]+a[i-2]+1-a[i-1];
}
else{
pref[i] = pref[i-1];
}
}
vector<int>suff(n+1,0);
for(int i = n;i>=2;i--){
if(a[i-1]<=a[i]+1){
suff[i] = suff[i+1]+a[i]+1-a[i-1];
}
else{
suff[i] = suff[i+1];
}
}
int ans = 0;
for(int i = 1;i<=n;i++){
ans = min(ans,max(pref[i],suff[i]));
}
cout<<ans;
}
