what the wrong with this code ? :

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int n,t;
vector <pair<int,int> > cow;

int main(){
    cin >>n >>t;
    for (int i=0;i<n;i++){
        int a,b;
        cin >>a >>b;
        cow.push_back(make_pair(a,b));
    }
    sort (cow.begin(),cow.end());
    int e=1,mx=-1,c=0,p=0;
    while (e<=t && p<cow.size()){
        while (cow[p].first<=e){
            mx=max(mx,cow[p].second);
            p++;
        }
        if (mx<e){
            cout <<-1<<endl;
            return 0;
        }
        c++;
        e=mx+1;
    }
    if (e==t+1)
        cout <<c<<endl;
    else
        cout <<-1<<endl;
    return 0;
}

it's better to give me e good test case and don't say my bug! tnx ! :)