#include<iostream.h>
#include<fstream.h>
int main()
{
long teach[25005],stud[200005];
long i,j,kol=0,k,l,r,n,m;
cin>>n;
for(i=1;i<=n;i++)cin>>teach[i];
cin>>m;
for(i=1;i<=m;i++)
{
    cin>>stud[i];
    l=1;r=n;
    for(j=1;j<=m;j++)
    {
    if(stud[i]<teach[1])break;else
    {
    k=(l+r)/2;
    if(stud[i]>teach[k])l=k+1;else
    if(stud[i]<teach[k])r=k-1;else {kol++;break;}
    }
    }
}cout<<kol;
return 0;
}

M <=1000000, but

for(j=1;j<=m;j++)
{
if(stud[i]<teach[1])break;else
{
k=(l+r)/2;
if(stud[i]>teach[k])l=k+1;else
if(stud[i]<teach[k])r=k-1;else {kol++;break;}
}
}
It's not correct

You should not to store student years
Just read next student year and find it
with binary search in teach[]