#include <bits/stdc++.h>
using namespace std;
#define int long long
mt19937_64 RNG(chrono::steady_clock::now().time_since_epoch().count());
void Solve() {
int n;
cin >> n;
int a[n + 1];
for(int i = 1; i <= n; i++) {
cin >> a[i];
}
int left[n + 1], right[n + 1];
right[n] = 0, left[1] = 0;
for(int i = 2; i <= n; i++) {
left[i] = left[i - 1] + max(a[i - 1] + 1 - a[i], 0LL);
}
for(int i = n - 1; i > 0; i--) {
right[i] = right[i + 1] + max(a[i + 1] + 1 - a[i], 0LL);
}
int ans = min(left[n], right[1]);
for(int i = 2; i < n; i++) {
ans = min(ans, max(left[i], right[i]));
}
cout << ans << "\n";
}
int32_t main() {
auto begin = std::chrono::high_resolution_clock::now();
ios_base::sync_with_stdio(0);
cin.tie(0);
int t = 1;
//cin >> t;
while(t--) {
Solve();
}
auto end = std::chrono::high_resolution_clock::now();
auto elapsed = std::chrono::duration_cast<std::chrono::nanoseconds>(end - begin);
cerr << "Time measured: " << elapsed.count() * 1e-9 << " seconds.\n";
return 0;
}
