Unable to swap even number of elements in linked-list

I am writing code to swap elements in linked list pairwise,my code works fine with odd number of element but is not working with even number of elements

Input-1->2->3->4->5->6->7

Output-2->1->4->3->6->5->7

 void sort()
 {
     node *ptr=head;
     node *ptr1=head;
     int temp;
     while(ptr->next!=NULL)
     {
         temp=(ptr->next)->info;
         (ptr->next)->info=ptr->info;
         ptr->info=temp;

         //This statement is not working with even number of elements as
         //when it reaches last element it is going beyond the list and 
         //I am unable to rectify how to rectify this problem
         ptr=(ptr->next)->next;
     }
 }