how did you do it with set ? can you please give me your code and idea.

this is my code.but its getting WA :(

#include<bits/stdc++.h>
using namespace std;
int ks,kase,n,seen;
pair<int,int>pos[40010];
set<int>S;
int main()
{
    scanf("%d",&kase);
    for(ks=1;ks<=kase;ks++)
    {
        scanf("%d",&n);
        S.clear();
        seen=0;
        for(int i=0;i<n;i++)
        {
            scanf("%d %d",&pos[i].first,&pos[i].second);
            S.insert(pos[i].first);
            S.insert(pos[i].second);
        }
        set<int>::iterator it,jt;
        for(int i=n-1;i>=0;i--)
        {
            it=S.lower_bound(pos[i].first);
            jt=S.lower_bound(pos[i].second);
            if(it!=jt )
            {
                seen++;
                S.erase(it,jt);
            }
        }
        printf("%d\n",seen);
    }
}