How to convert 12 Hours to 24 Hours

def convert24(str1):
    if str1[-2:] == "AM" and str1[:2] == "12":
        return "00" + str1[2:-2]
 
    elif str1[-2:] == "AM":
        return str1[:-2]
    elif str1[-2:] == "PM" and str1[:2] == "12":
        return str1[:-2] 
    else:
        return str(int(str1[:2]) + 12) + str1[2:8]     
print(convert24("08:05:45 PM"))