p=input().rstrip()
a=list(p)
m=''
l=[]
p=1
s=1
e=1
for i in range(0,len(a)):
    if(a[i].isdigit()==True):
        m=m+a[i]
    elif(a[i] is ' '):
        l.append(m)
        m=''
n=int(l[0])
k=int(l[1])
x=int(m)
for i in range(1,n+1):
    s=s*i
for i in range(1,n-k+1):
    p=p*i
for i in range(1,k+1):
    e=e*i
sum=s//(p*e)-1
print(sum)