Write a function to find the product of first even and odd number of a given list.
def mul_even_odd(list1):
    first_even = next((el for el in list1 if el%2==0),-1)
    first_odd = next((el for el in list1 if el%2!=0),-1)
    return (first_even*first_odd)
assert mul_even_odd([1,3,5,7,4,1,6,8])==4 assert mul_even_odd([1,2,3,4,5,6,7,8,9,10])==2 assert mul_even_odd([1,5,7,9,10])==10