#include <bits/stdc++.h>
using namespace std;
#pragma GCC target ("avx2")
#pragma GCC optimization ("Ofast")
#pragma GCC optimization ("unroll-loops")
typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> ii;
typedef vector<ii> vii;
typedef long double ld;
typedef pair<ll, ll> pll;
#define FOR(i, a, b) for(int i = a; i < b; i++)
#define ROF(i, a, b) for(int i = a; i >= b; i--)
#define ms memset
#define pb push_back
#define fi first
#define se second
ll MOD = 1000000007;
ll MOD1 = 998244353;
ll power(ll base, ll n){
if (n == 0) return 1;
if (n == 1) return base;
ll halfn = power(base, n/2);
if (n % 2 == 0) return (halfn * halfn) % MOD;
return (((halfn * halfn) % MOD) * base) % MOD;
}
ll inverse(ll n){
return power(n, MOD-2);
}
ll add(ll a, ll b){
return (a+b) % MOD;
}
ll mul(ll a, ll b){
a %= MOD;
return (a*b) % MOD;
}
ll gcd(ll a, ll b){
if (a == 1) return 1;
if (a == 0) return b;
return gcd(b%a, a);
}
ld pi = 3.141592653589793238;
void solve(){
int n; cin >> n;
vector<ll> a(n); FOR(i,0,n) cin >> a[i];
vector<ll> pre(n), suf(n);
ll mx = a[0];
FOR(i,1,n){
pre[i] = pre[i-1] + max((ll)0,mx+1-(a[i]+pre[i-1]));
mx=max(mx+1,a[i]+pre[i]);
}
mx = a[n-1];
ROF(i,n-2,0){
suf[i] = suf[i+1] + max((ll)0, mx+1-(a[i]+suf[i+1]));
mx=max(mx+1,a[i]+suf[i]);
}
//FOR(i,0,n){cout << pre[i] << " " << suf[i] << '\n';}
ll mn = max(pre[0],suf[0]);
FOR(i,1,n) mn=min(mn,max(pre[i],suf[i]));
cout << mn << '\n';
return;
}
int main() {
cout << fixed << setprecision(8);
ios::sync_with_stdio(false);
if (fopen("input.txt", "r")) {
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
}
int TC = 1;
//cin >> TC;
FOR(i, 1, TC+1){
//cout << "Case #" << i << ": ";
solve();
}
}
