def standard_monomial(D, low=None, east=1):                      
    n = len(D)                         
    A = [ i+1 for i,s in enumerate(D) if s == 1-east ]
    low = [ i+1 for i,s in enumerate(low) if s == 1-east ]
    if low is None:
        low = [1 .. len(A)]
    low = [ i for i in [1 .. n] if i not in low ]
    vals = []
    j = 0
    blocker = low + [Infinity]
    for i in [1 .. n]:
        if i not in A:
            if i < blocker[0]:
                j += 1
            else:
                blocker.pop(0)
        else:
            if j > 0:
                j -= 1
                vals.append(i)
                blocker.pop(0)
    return tuple(vals)

def statistic(D):
    n = len(D)//2
    return len(standard_monomial(D, tuple([1,0]*n), east=1))