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gistfile1.txt

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int min_coupons(vector<int>& arr) {
    int n = arr.size();
    vector<int> dp(n, 1);

    for (int i = 1; i < n; ++i) {
        if (arr[i] >= arr[i - 1]) {
            dp[i] = dp[i - 1];
        } else {
            dp[i] = dp[i - 1] + arr[i - 1] - arr[i] + 1;
            arr[i] = arr[i - 1] + 1;
        }
    }

    return dp[n - 1] - 1;
}

int main() {
    int n;
    cin >> n;
    vector<int> arr(n);
    for (int i = 0; i < n; ++i) {
        cin >> arr[i];
    }

    cout << min_coupons(arr) << endl;
    return 0;
}