#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ff first
#define ss second
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
const int N = 200005;
const ll MOD = 1000000007;
const ll INF = 0x3f3f3f3f3f3f3f3f;
ll a[N], d[N];
ll l[N], r[N];
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n;
cin>>n;
for(int i = 1; i <= n; ++i)
cin>>a[i];
for(int i = 1; i < n; ++i)
d[i] = a[i + 1] - a[i];
ll t = 0;
for(int i = 1; i < n; ++i){
l[i] += t;
t += max(0LL, 1 - d[i]);
}
t = 0;
for(int i = n - 1; i >= 1; --i){
r[i] += t;
t += max(0LL, d[i] + 1);
}
ll ans = 1'000'000'000'000'000'000;
for(int i = 1; i <= n; ++i)
ans = min(ans, max(l[i], r[i]));
cout<<ans;
}
