#include <bits/stdc++.h>
using namespace std;
#define ll long long
int n, a[310000];
ll pre[310000] = {0}, suf[310000] = {0};
int main () {
ios_base::sync_with_stdio(false); cin.tie(0);
cin >> n;
for (int i = 1; i <= n; i++) cin >> a[i];
a[0] = a[n + 1] = -1;
for (int i = 1; i <= n; i++) {
pre[i] = pre[i - 1];
if (a[i] <= a[i - 1]) pre[i] += a[i - 1] + 1 - a[i];
}
for (int i = n; i >= 1; i--) {
suf[i] = suf[i + 1];
if (a[i] <= a[i + 1]) suf[i] += a[i + 1] + 1 - a[i];
}
ll ans = min(pre[n], suf[1]);
for (int i = 1; i <= n; i++) ans = min(ans, max(pre[i - 1], suf[i]));
cout << ans << '\n';
return 0;}
