3.4.2

def convert_2_to_10(n):
    dec = 0
    multiplier = 0
    for b in n[::-1]:
        dec += int(b)*2**multiplier
        multiplier +=1
    return dec
        #print str(b) + " " + str(dec) + " ||",

print convert_2_to_10("10")       # 2
print convert_2_to_10("1000")     # 8
print convert_2_to_10("1010")     # 10
print convert_2_to_10("11111")    # 31
print convert_2_to_10("101010")   # 42