#include <iostream>
#include <fstream>
#include <cmath>
#include <algorithm>
using namespace std;
#define min(a,b) (a<b?a:b)
#define max(a,b) (a>b?a:b)
int main(){
    long long n,k,x;
    cin>>n>>k>>x;
    cout<<(k-1)*k/2;
    return 0;
}