#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx,popcnt,sse4,abm")
#include <bits/stdc++.h>
using namespace std;
#ifdef WAIMAI
#define debug(HEHE...) cout << "[" << #HEHE << "] : ", dout(HEHE)
void dout() {cout << '\n';}
template<typename T, typename...U>
void dout(T t, U...u) {cout << t << (sizeof...(u) ? ", " : ""), dout(u...);}
#else
#define debug(...) 7122
#endif
#define ll long long
#define Waimai ios::sync_with_stdio(false), cin.tie(0)
#define FOR(x,a,b) for (int x = a, I = b; x <= I; x++)
#define pb emplace_back
#define F first
#define S second
const int SIZE = 2e5 + 5;
int n;
int a[SIZE], d[SIZE];
ll ans, pre[SIZE], suf[SIZE];
void solve() {
cin >> n;
FOR (i, 1, n) cin >> a[i];
FOR (i, 1, n) d[i] = a[i] - a[i - 1];
FOR (i, 1, n) pre[i] = pre[i - 1] + max(0, 1 - d[i]);
for (int i = n; i >= 1; i--) suf[i] = suf[i + 1] + max(0, d[i] + 1);
ans = LLONG_MAX;
FOR (i, 1, n) ans = min(ans, max(pre[i], suf[i + 1]));
cout << ans << '\n';
}
int main() {
Waimai;
solve();
}
