#include <bits/stdc++.h>
using namespace std;
#define LL long long
int main() {
ios::sync_with_stdio(false), cin.tie(0);
int n;
cin >> n;
long long ans = 0;
vector<long long> a(n + 5);
vector<long long> l(n + 5);
vector<long long> r(n + 5);
for (int i = 1; i <= n; i++) {
cin >> a[i];
}
for (int i = 2; i <= n; i++) {
l[i] = l[i - 1];
l[i] += max(0ll, a[i - 1] - a[i] + 1);
}
for (int i = n - 1; i > 0; i--) {
r[i] = r[i + 1];
r[i] += max(0ll, a[i + 1] - a[i] + 1);
}
ans = 1e18;
for (int i = 1; i <= n; i++) {
ans = min(ans, max(l[i], r[i]));
}
cout << ans << endl;
return 0;
}
