Why is the invalid letter not showing?

[Robrick]

I wanna know why the program doesnt show "Invalid Letter Entered" when i enter any letter other than A S D or M

//Processing the data
	if (letter== 'A'||'S'||'M'||'D')// checking Add, subtract, multiply or divide
	{
		if (letter== 'A')//Adding the integers
			cout<<"Adding two integers = "<<first + second<<endl;
		else if (letter== 'S')//Subtracting the integers
		{
			if (first < second)
			cout<< "Subtracting two integers = "<<second - first<<endl;
			else if (first > second)
			cout<< "Subtracting two integers = "<<first - second<<endl;
		}
	
		else if (letter== 'M')//Multiplying the integers
			cout<<"Multiplying two integers = "<<first * second<<endl;
		else if (letter== 'D')//Dividing the integers
		{
			if (first > second)
			cout<< "Dividing two integers = "<<first/second<<endl;
			else if (first < second)
			cout<< "Dividing two integers = "<<second/first<<endl;
		}
	}
	else
		cout<<"Invalid Letter Entered"<<endl;
	return 0;

[Rabbit]

It should be

if (letter== 'A' || letter== 'S'|| letter== 'M'|| letter== 'D')