I just can't see my mistake! I have considered the zero and 1..9
cases, and output -1 for numbers that have a prime divisor greater
that 7, but it's stil WA ???

 Program Digits_Product(input,output);
 Var N,i,j,k:longint;
     A:array[1..4]of 2..7;
     B:array[1..255]of word;
     Q:string;

 Procedure Sort(k:longint);
    var i,j:byte;
        p:word;
    begin
      for i:=1 to K-1 do
        for j:=i+1 to K do
          if B[i]>B[j] then
            begin
              p:=B[i];
              B[i]:=B[j];
              B[j]:=p
            end
    end;

 Begin
   readln(N);
   for i:=1 to 255 do B[i]:=1;
   A[1]:=2;
   A[2]:=3;
   A[3]:=5;
   A[4]:=7;
   case N of
        0:begin writeln(10); halt end;
     1..9:begin writeln(N); halt end
     else Q:='-1';
   end;
   i:=1;
   for j:=1 to 4 do
     while N mod A[j]=0 do
       begin
         B[i]:=A[j];
         N:=N div A[j];
         i:=i+1
       end;
   if N<>1 then
     begin
       writeln(Q);
       halt
     end;
   k:=i;
   for i:=1 to k-1 do
     for j:=i+1 to k do
        if B[i]*B[j]<=9 then
           begin
             B[i]:=B[i]*B[j];
             B[j]:=1;
           end;
   Sort(k);
   Q:='';
   for i:=1 to k do
      if B[i]<>1 then Insert(chr(B[i]+48),Q,length(q)+1);
   writeln(Q);
 End.