#include <bits/stdc++.h>
#define all(i) (i).begin(), (i).end()
using namespace std;
#define debug(args) cout << '[' << #args << "] is [", DEBUG(false, args...), cout << endl
template<typename T>
void DEBUG(bool _sp, T i) {
if (_sp)
cout << ' ';
cout << i;
}
template<typename T, typename ...U>
void DEBUG(bool _sp, T i, U ...j) {
if (_sp)
cout << ' ';
cout << i;
DEBUG(j...);
}
typedef long long ll;
typedef pair<int, int> pi;
typedef double dd;
const int inf = 0x3f3f3f3f, lg = 20;
const ll mod = 1e9 + 7, INF = 0x3f3f3f3f3f3f3f3f;
signed main() {
ios::sync_with_stdio(0), cin.tie(0);
int n;
cin >> n;
int a[n];
for (int &i : a)
cin >> i;
vector<ll> d;
d.push_back(inf);
for (int i = 1; i < n; ++i)
d.push_back(a[i] - a[i - 1]);
d.push_back(-inf);
vector<ll> pre(n + 1), suf(n + 1);
pre[0] = suf[n] = 0;
for (int i = 1; i <= n; ++i)
pre[i] = pre[i - 1] + max(0ll, 1 - d[i]);
for (int i = n - 1; ~i; --i)
suf[i] = suf[i + 1] + max(0ll, d[i] + 1);
ll ans = INF;
for (int i = 0; i < n; ++i)
ans = min(ans, max(pre[i], suf[i + 1]));
cout << ans;
}
