#include <cstdio>
#include <iostream>
#include <cmath>
#include <ctime>
#include <vector>
#define volim push_back

using namespace std;

vector< pair<int,int> > latinski;
long long int sol;

int main (){
    int n, m; cin >> n >> m;
    int x, y;
    for(int i=0; i<n; i++){
        cin >> x >> y;
        latinski.volim(make_pair(x, y));
    }
    vector<int>lingua;
    int k=1;
    while(k){
        k=0;
        for(int i=n-1; i>0; i--){
            if(latinski[i].first > latinski[i-1].first){
                swap(latinski[i], latinski[i-1]);
                k=1;
            }
        }
    }
    int dicunt = latinski[0].second;
    sol+=latinski[0].second;
    
    for(int i=1; i<n; i++){
        if(dicunt<latinski[i].second) sol+=latinski[i].second-dicunt;
        dicunt=latinski[i].second;
        }
        
        
    cout << sol << endl; 
 
    return 0;
}