#include <bits/stdc++.h>
#define int long long
#define ff first
#define ss second
#define all(x) x.begin(),x.end()
#define pb push_back
using namespace std;
signed main(){
ios_base::sync_with_stdio();
cin.tie(0);cout.tie(0);
int n;
cin>>n;
vector <int> a(n);
for(int i=0;i<n;i++){
cin>>a[i];
}
vector <int> pr(n),sf(n);
for(int i=1;i<n;i++){
pr[i]=pr[i-1];
if(a[i-1]>=a[i])pr[i]+=a[i-1]-a[i]+1;
}
for(int i=n-2;i>=0;i--){
sf[i]=sf[i+1];
if(a[i+1]>=a[i])sf[i]+=a[i+1]-a[i]+1;
}
int ans=1e18;
for(int i=0;i<n;i++){
ans=min(ans,max(sf[i],pr[i]));
}
cout<<ans;
}
