#include <bits/stdc++.h>
using namespace std;
#define ll long long
int main() {
int n;
cin >> n;
int a[n + 1], diff[n + 1];
a[0] = 0;
for (int i = 1; i <= n; i++) {
cin >> a[i];
diff[i] = a[i] - a[i - 1];
}
// make diff positive followed be negative
ll pre[n + 2], suf[n + 2];
pre[0] = 0;
for (int i = 1; i <= n; i++) {
pre[i] = pre[i - 1] + max(1 - diff[i], 0);
}
suf[n + 1] = 0;
for (int i = n; i >= 1; i--) {
suf[i] = suf[i + 1] + max(diff[i] - (-1), 0);
}
ll ans = 1e18;
for (int i = 1; i <= n; i++) {
ans = min(ans, max(pre[i], suf[i + 1]));
}
cout << ans << endl;
}
