N,K = (int(n) for n in input().split(" "))
ans = 0
if N == 1:
    print(1)
elif K != 1:

    for num in range(1,N+1):
        if num == 1:
            ans += 1
        elif num <= K:
            pass
        else:
            ans += (N - num) + 1
            
    print(ans)
else:
    for num in range(2,N+1):
        if num == 2:
            pass
        else:
            ans += (N - num) + 1
    print(ans)