I use BFS in the program
#include<iostream.h>
long x[2000],y[2000];
int queue[2000];
int parent[2000];
int path[2000];
int in[2000];
void main()
{

 int n;
 int i,j,tot,current,step;
 int temp;
 long need;
 cin>>n;
 for(i=1;i<=n;i++){
   in[i]=0;
   cin>>x[i]>>y[i];
 }
 cin>>need>>x[0]>>y[0];

 queue[0]=0;
 current=0;
 tot=1;
 while(current<tot){
     if(x[queue[current]]==need||y[queue[current]]==need) break;
     for(i=1;i<=n;i++){
         if(!in[i]){
             if((x[i]==x[queue[current]])||(x[i]==y[queue
[current]])||(y[i]==x[queue[current]])||(y[i]==y[queue[current]])){
               queue[tot]=i;
               parent[i]=queue[current];
               in[i]=1;
               tot++;
             }
         }
     }
     current++;
 }

 if(x[queue[current]]==need||y[queue[current]]==need){
     step=0;
     temp=queue[current];
     while(1){
      if(temp==0) break;
      step++;
      path[step]=temp;
      temp=parent[temp];
     }
     cout<<step<<endl;
     for(j=step;j>=1;j--){
      cout<<path[j]<<endl;
     }
 }else{
  cout<<"IMPOSSIBLE"<<endl;
 }
}

when i change

if((x[i]==x[queue[current]])||(x[i]==y[queue[current]])
  ||(y[i]==x[queue[current]])||(y[i]==y[queue[current]]))
{}

into

if((x[i]==x[queue[current]])||(x[i]==y[queue[current]])
{}

i get ac.
i did not read the problem carefully
Anyone who got wrong answer should pay attention to it

Thanks! But where it is mentioned in the problem statement???

Another THANKS from me, but really where is that mentioned in the problem's statement ?

THANKS, man!

Well, there is a sentence in text:

"The next K lines contain the number of the route of the corresponding bus and the number on the other side of its plate."

But it is not explicitly said that first number in line is route number and that second number is from the other side of plate...

Anyway, the point is that you can change your plate with driver i (which has plate x[i],y[i]) iff one of the numbers on your plate is x[i] (not x[i] OR y[i]).