mail = "ahmet@asd.sda"
def is_alphabetical(x):
    alphabet = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"] 
    try:
        alphabet.index(x.lower())
        a = True
    except:
        a = False
    return a

def is_mailnumeric(x):
    alphabet = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x",
                "y","z","0","1","2","3","4","5","6","7","8","9",".","-","_"] 
    try:
        alphabet.index(x.lower())
        a = True
    except:
        a = False
    return a
at_pos = 0
dot_pos = 0 
def find_at_position(mail):
    global at_pos
    for p in mail:
        at_pos = at_pos + 1 
        if p == "@":
            return at_pos
def find_dot_position(mail):
    global dot_pos
    for p in mail:
        dot_pos = dot_pos + 1 
        if p == ".":
            return dot_pos
if not(isinstance(find_at_position(mail), int)):
    print ("No @ found or mail invalid ")
if not(isinstance(find_dot_position(mail), int)):
    print ("No . found or mail invalid")
m = -1
while m == -1:
    if not(is_alphabetical(mail[0])):
        print("First letter problem mail invalid")
        exit()
    m = m + 1
ap = find_at_position(mail)
dp = find_dot_position(mail)
while m < (ap-1):
    m = m + 1
    if not(is_mailnumeric(mail[m])):
        print("Invalid Email")
        exit()
m = m + 1 
while m > (ap-1) and m < (dp-1):
    m = m + 1
    if is_alphabetical(mail[m])== False:
        print("Invalid email problem with domain")
        exit()
while m > (dp-1) and m <= len(mail):
    n = 0
    n = n + 1
    if n > 3 or not(is_alphabetical(mail[m])):
        print ("Invalid email problem with extension")
        exit()
print ("Valid email")