My solution is:



#include <iostream>
//#include <conio.h>
using namespace std;
int prepod[15000], n;
int binsearch(int num)
{
    int l = 0, r = n - 1, m;
    while (r - l > 1)
    {
        int m = (r + l) / 2;
        if (prepod[m] == num)
          return 1;
        if (prepod[m] > num)
            l = m;
        if (prepod[m] > num)
            r = m;
    }
    if ((prepod[l] != num) && (prepod[r] != num))
      return 0;
    else
      return 1;
}
int main()
{
    int m, student, s=0;
    cin>>n;
    for (int i = 0; i < n; i++)
      scanf ("%i", &prepod[i]);
    cin>>m;
    for (int i = 0; i < m; i++)
    {
        scanf ("%i", &student);
        s+=binsearch (student);
    }
    cout<<s;
    //getch();
    return  0;
}


And it has time O(mlogn) and it is about 9e+6.