this is my checker of this problem, it can't be wrong ,but WA on test 8

    if(i>=n||a[i]!=k)
    {
        while(!(n==5&&k==5))
           puts("orz");
       puts("-1");
        }
    else
    {
        printf("%d\n",cnt_ans);
        while(cnt_ans>3*n)
           puts("orz");
        for(i=0;i<cnt_ans;i++)
           printf("%d %d\n",ax[i],ay[i]);
        for(i=0;i<n;i++)
            a[i]=1;
        for(i=0;i<cnt_ans;i++)
        {
            int id1=ax[i]-1,id2=ay[i]-1;
            while(id1>=n||id1<0||id2>=n||id2<0||id1==id2)
               puts("orz");
            a[id1]-=a[id2];
            a[id2]*=2;
            while(a[id1]<0)
               puts("orz");
        }
        for(i=0;i<n;i++)
        {
            if(a[i]==k)
               break;
        }
        while(i!=0)
           puts("orz");
    }