printmystring = ""
mystring = input("Enter the sentence you wish to be an acronym: ")
mystring_length = len(mystring)
for i in range (0,mystring_length):
if mystring[i].isupper():
print(mystring[i] ,end="")
print(".",end="")
私の入力は「参考までに」です
出力は参考までに