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')