#include <fstream>
using namespace std;
ifstream fin ("orase.in");
ofstream fout ("orase.out");
int v[50001],v2[50001];
int main()
{
    int m,n,x,y,i,lmax,cmax;
    fin >>m>>n;
    for (i=1;i<=m;++i)
    {
        fin >>x>>y;
        if (x>v[y])
        {
            v[y]=x;
        }
        else
        {
            if (x>v2[y])
                v2[y]=x;
        }
    }
    for (i=1;i<=m;++i)
    {
        if (v[i]+cmax>lmax)
            lmax=v[i]+cmax;
        if (v[i]>cmax)
            cmax=v[i];
        if (v2[i]+cmax>lmax)
            lmax=v2[i]+cmax;
    }
    fout <<lmax;
    fin.close ();
    fout.close ();
    return 0;
}