#include <bits/stdc++.h>
#define int long long
using namespace std;
void solve() {
int n;
cin >> n;
vector <int> val(n);
for (int i = 0; i < n; ++i) {
cin >> val[i];
}
vector <int> prev(n, 0), suf(n, 0);
for (int i = 1; i < n; ++i) {
prev[i] += prev[i - 1];
prev[i] += max(0LL, val[i - 1] - val[i] + 1);
}
for (int i = n - 2; i >= 0; --i) {
suf[i] += suf[i + 1];
suf[i] += max(0LL, val[i + 1] - val[i] + 1);
}
int ans = 2e18;
for (int i = 0; i < n; ++i) {
ans = min(ans, max(prev[i], suf[i]));
}
cout << ans << '\n';
}
signed main() {
cin.tie(0);
ios::sync_with_stdio(false);
int t = 1;
// cin >> t;
for (int i = 0; i < t; ++i) {
solve();
}
}
