#include <bits/stdc++.h>
#define ll long long
using namespace std;
const ll N = 1e6, mod = 1e9 + 7, inf = 1e18;
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int n;
cin >> n;
vector<ll> a(n + 2, 0);
for (int i = 1; i <= n; i++) {
cin >> a[i];
}
vector<ll> k(n + 2, 0), d(n + 2, 0), d1(n + 2, 0);
ll ans = 0;
vector<ll> b = a;
for (int i = n; i >= 1; i--) {
if (b[i] + d[i + 1] > b[i + 1]) {
k[i] = ans;
d[i] = d[i + 1];
b[i] = b[i] + d[i + 1];
}
else {
b[i] = b[i] + d[i + 1];
ll e = b[i + 1] - b[i] + 1 + d[i + 1];
d[i] = max(e, d[i + 1]);
ans += d[i] - d[i + 1];
b[i] = b[i + 1] + 1;
k[i] = ans;
}
}
b = a;
ll mnd = int64_t(1e18);
ans = 0;
for (int i = 1; i <= n; i++) {
if (b[i] + d1[i - 1] > b[i - 1]) {
d1[i] = d1[i - 1];
b[i] = b[i] + d1[i - 1];
}
else {
b[i] = b[i] + d1[i - 1];
ll e = b[i - 1] - b[i] + 1 + d1[i - 1];
d1[i] = max(e, d1[i - 1]);
ans += d1[i] - d1[i - 1];
b[i] = b[i - 1] + 1;
}
mnd = min(mnd, max(ans, k[i]));
}
cout << mnd << '\n';
}
