Simple boolean stuck am i not seeing it right

Simple boolean stuck am i not seeing it right

Sorry maybe i am being slow here today

for the following  if i enter a or b or c they all go to the IF PART

am i not saying if not a or b go into the IF PART

if ((input.Single_Line_1  !=  "a")  ||  (input.Single_Line_1  !=  "b"))
{
    alert "IF PART";
    cancel submit;
}
alert "NO IF PART";
cancel submit;