#include <iostream>
#include <algorithm>

using namespace std;

bool cmp(pair<int,int> a , pair<int,int>b)
{
return a.first>b.first;
};

void isp();
pair<int,int> D[100100];
int n,m;
int main()
{
    cin>>n>>m;
    for (int i =0; i<n; i++)
    {
        int tmp, tmp1;
        cin>>tmp>>tmp1;
        D[i].first = tmp;
        D[i].second = tmp1;
    }
    if (n==1) {cout<<D[0].second; return 0;}
    sort(D, D+n, cmp);
    
    D[n].second = 0;
    //isp();
    
    int rj=0, akt = 0;
    for (int i =0; i<=n; i++)
    {
        if (akt>D[i].second) 
            rj+=(akt - D[i].second);
            
        akt = D[i].second;
    }
    cout<<rj;

}

void isp()
{
     cout<<"***Ispis liste D od 0.clana***"<<endl;
for (int i =0; i<=n; i++)
cout<<D[i].first<<" "<<D[i].second<<endl;     
cout<<"***zavrsetak ispisa***"<<endl;
}