/// @author Camillus
#include "bits/stdc++.h"
#define int long long
using namespace std;

signed 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];
    }

    vector<int> left(n), right(n);
    for (int i = 1; i < n; i++) {
        left[i] = left[i - 1] + max(0ll, a[i - 1] - a[i] + 1);
    }

    for (int i = n - 2; i >= 0; i--) {
        right[i] = right[i + 1] + max(0ll, a[i + 1] - a[i] + 1);
    }

    int ans = INT32_MAX;

    for (int i = 0; i < n; i++) {
        ans = min(ans, max(left[i], right[i]));
    }

    cout << ans << '\n';
    return 0;
}