n=int(input())
ans=1
if n==0:
    print(0)
    exit()

for i in range(n//10 - (n//10)%2,n//2+1):
    for j in range(i//10,i+1):
        if n < i*2 + j*2:
            continue
        ans=max(i*j,ans)
            
print(ans)