#include<iostream>
#include<algorithm>
#include<cstring>

using namespace std;

const int N = 1010;

int n,m;
int a[N];

int main()
{
    cin>>n>>m;
    for(int i = 1; i<=n; i++) cin>>a[i];
    a[n+1] = m;
    int k = 0;
    int sum = 0;
    for(int i = 0; i<=n; i++)
    {
        for(int j = a[i]; j<a[i+1]; j++)
            sum+=k;
        k++;
    }
    cout<<sum;
    return 0;
}