confusion of halting problem

Question

Show that the following problem is solvable.Given two programs with their inputs and the knowledge that exactly one of them halts, determine which halts.

lets P be program that determine one of the program will be halted.

P(Program x,Program y){

    if(x will be halted)

          then return 1;

    else

           then return 2;
}

since we know that exactly one of them will be halted,if 1 then program x will be halted.Otherwiae program y will be halted.

then we construct a new program call D

D(X,Y){

     if(P(X,Y) == 2)

         D will halt;

      else

         while(1)//D will not halt;

  }

lets S be aritbrary program.

So if we have D(D,S)

if D will halt then D will not halt

if D will not halt then D will halt

It impiles a contradiction same as halting problem.

But the question stated that it is solvable.