#include "bits/stdc++.h"
#define in std::cin
#define out std::cout
#define rep(i,N) for(LL i=0;i<N;++i)
typedef long long int LL;

int main()
{
	LL N, K;
	in >> N >> K;

	LL max = N * (N + 1) / 2;
	LL min = (N - K) * (N - K + 1) - max;
	out << (max - min) / 2 + 1 << std::endl;
}