#include <iostream>
#include <algorithm>
using namespace std;
const int N = 210;
int a[N];
int s[N];
int main()
{
    int n, m;
    cin >> n >> m;
    for (int i = 1; i <= n; i ++ ) cin >> a[i];
    int sum = 0;
    for (int i = 1; i < n; i ++)
        s[i] = a[i + 1] - a[i];
    for (int i = 1; i < n; i ++ ) 
        sum += s[i] * i;
    sum += n * (m - a[n]);
    cout << sum << endl;
    return 0;
}