#include <bits/stdc++.h>
using namespace std;
#define endl '\n'
#define dl double
#define int long long
#define ul uint_fast32_t
#define ll int_fast64_t
#define dll long double
#define ull uint_fast64_t
#define spektar this_thread::sleep_for(chrono::milliseconds(50))
void solve(){
int n;
cin >> n;
vector<ll> k(n);
for(auto& i:k) cin >> i;
vector<ll> k1=k,k2=k;
vector<ll> kk1(n);
vector<ll> kk2(n);
kk1[0]=0;
kk2[n-1]=0;
ll max1=0;
for(int i=1; i<n; i++){
ll s1=max(k1[i],k1[i-1]+1)-k1[i];
k1[i]=max(k1[i],k1[i-1]+1);
kk1[i]=s1;
}
for(int i=n-2; i>=0; i--){
ll s1=max(k2[i],k2[i+1]+1)-k2[i];
k2[i]=max(k2[i],k2[i+1]+1);
kk2[i]=s1;
}
vector<ll> pref1(n+1),pref2(n+1);
pref1[0]=0;
pref2[n]=0;
for(int i=0; i<n; i++){
pref1[i+1]=pref1[i]+kk1[i];
}
for(int i=n-1; i>=0; i--){
pref2[i]=pref2[i+1]+kk2[i];
}
ll min1=1000000000000000000;
for(int i=0; i<n; i++){
min1=min(min1,min(max(max(kk1[i],kk2[i]),pref1[i])+pref2[i+1],max(pref2[i+1],max(kk1[i],kk2[i]))+pref1[i]));
}
cout << min1 << endl;
}
signed main(){
//freopen("input.txt","r",stdin);
//freopen("output.txt","w",stdout);
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
int t=1;
//cin >> t;
while(t--){
solve();
}
}
