#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 5;
int n;
long long a[maxn], l[maxn], r[maxn], lans[maxn], rans[maxn];
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) scanf("%I64d", &a[i]);
  for (int i = 1; i <= n; i++) {
    l[i] = max(a[i], l[i - 1] + 1);
    lans[i] = l[i] - a[i] + lans[i - 1];
  }
  for (int i = n; i >= 0; i--) {
    r[i] = max(a[i], r[i + 1] + 1);
    rans[i] = r[i] - a[i] + rans[i + 1];
  }
  long long ans = 1e18;
  for (int i = 1; i <= n; i++)
    ans =
        min(ans, lans[i - 1] + rans[i + 1] + max(l[i], max(r[i], a[i])) - a[i]);
  printf("%I64d\n", ans);
  return 0;
}
