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

Code:
//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
...