#include<iostream>
using namespace std;
int main()
{
	int n,k,x;
	cin>>n;
	cin>>k;
	cin>>x;
	int first=n-2;
	int sum=0,i;
	k=k-1;
	for(i=0;k>0;i++)
	{
		sum+=k;
		k=k-1;
	}
	cout<<sum;
	return 0;
}