#include <bits/stdc++.h>
using namespace std;
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n;
cin >> n;
vector<int> a(n);
for (int i = 0; i < n; i++) {
cin >> a[i];
}
int64_t l = 0, r = 0;
for (int i = 0; i < n - 1; i++) {
if (a[i] <= a[i + 1]) {
r += a[i + 1] - a[i] + 1;
}
}
int64_t ans = max(l, r);
for (int i = 0; i < n - 1; i++) {
if (a[i] <= a[i + 1]) {
r -= a[i + 1] - a[i] + 1;
}
if (a[i] >= a[i + 1]) {
l += a[i] - a[i + 1] + 1;
}
ans = min(ans, max(l, r));
}
cout << ans << '\n';
return 0;
}
