# include <bits/stdc++.h>
#define f first
#define s second
#define pb push_back
using namespace std;
const int N=1e6+5;
long long n,a[N],dp[N],pre,a1[N],dp1[N],a2[N],ans;
vector <long long> v;
int main() {
cin>>n;
for (int i=1; i<=n; i++) {
cin>>a[i];
a1[i]=a[i];
a2[i]=a[i];
}
for (int i=1; i<=n; i++) {
a1[i]+=pre;
//cout<<a1[i]<<" "<<a1[i-1]<<endl;
if(a1[i]<=a1[i-1]) {
pre+=abs(a1[i-1]-a1[i])+1LL;
a1[i]=a1[i-1]+1;
}
dp[i]=pre;
// cout<<a1[i]<<" ";
//cout<<dp[i]<<" ";
//cout<<a1[i]<<" ";
}
// cout<<endl;
//for (int i=1; i<=n; i++)
//cout<<dp[i]<<" ";
//cout<<endl;
pre=0;
for (int i=n; i>=1; i--) {
a2[i]+=pre;
if (a2[i]<=a2[i+1]) {
pre+=abs(a2[i+1]-a2[i])+1LL;
a2[i]=a2[i+1]+1;
}
dp1[i]=pre;
// cout<<dp1[i]<<" ";
}
// cout<<endl;
ans=min(dp[n],dp1[1]);
for (int i=1; i<n; i++) {
//cout<<dp[i]<<" "<<dp[i+1]<<endl;
long long anss=max(dp[i],dp1[i+1]);
if (a1[i]+anss==a2[i+1]+anss) ans=min(ans,max(dp[i],dp1[i+1])+1);
else
ans=min(ans,max(dp[i],dp1[i+1]));
}
cout<<ans<<endl;
// for (int i=1; i<=n; i++) {
// cout<<dp[i]<<" "<<dp1[i+1]<<endl;
//}
}
