#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int N = 2e5 + 5, inf = 2e9;
int n, d[N];
ll ans = 1e18, pf[N], sf[N];
int main() {
ios_base::sync_with_stdio(0); cin.tie(0);
// freopen(".INP", "r", stdin);
// freopen(".OUT", "w", stdout);
cin >> n;
for (int i=1;i<=n;++i) cin >> d[i];
for (int i=n;i>=1;--i) d[i] -= d[i - 1];
for (int i=2;i<=n;++i) pf[i] = pf[i - 1] + max(0, 1 - d[i]);
for (int i=n;i>=2;--i) sf[i] = sf[i + 1] + max(0, d[i] + 1);
for (int i=1;i<=n;++i) ans = min(ans, max(pf[i], sf[i + 1]));
cout << ans;
}
