#include <iostream>
#include <bits/stdc++.h>
#define ll long long
using namespace std;
const ll maxn = 2*1e5+5, INF = 4e18+9;
void solve(){
int n;
cin >> n;
vector<ll> a(n+2, 0), b, c;
for(int i = 1; i <= n; i++){
cin >> a[i];
}
vector<ll> pf(n+1, 0), sf(n+2, 0);
b = a;
for(int i = 1; i <= n; i++){
pf[i] = pf[i-1];
b[i] += pf[i-1];
if(b[i] <= b[i-1]){
pf[i] += b[i-1]-b[i]+1;
b[i] = b[i-1]+1;
}
}
c = a;
for(int i = n; i >= 1; i--){
sf[i] = sf[i+1];
c[i] += sf[i+1];
if(c[i] <= c[i+1]){
sf[i] += c[i+1]-c[i]+1;
c[i] = c[i+1]+1;
}
}
ll ans = INF;
for(int i = 1; i <= n+1; i++){
ll res = max(pf[i-1], sf[i]);
if(b[i-1] == c[i]){
res++;
}
ans = min(ans, res);
}
//cout << pf[1] << " " << sf[2] << "\n";
cout << ans;
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
solve();
}
