#include <bits/stdc++.h>
#include <fstream>

using namespace std;

ifstream f("orase.in");
ofstream g("orase.out");

long long int m , n , d , l , i , j , k , c1 , c2 , i1 , i2 , v[50005] , a[50005];

int main()
{
    f >> m >> n;

    for ( i = 1 ; i <= n ; i ++ )
    {
        f >> d >> l;

        v[i] = l;

        a[i] = d;
    }

    for ( i = 1 ; i <= n - 1 ; i ++ )
    {
        for ( j = i + 1 ; j <= n ; j ++ )
        if ( v[i] < v[j] )
        {
            swap ( v[i] , v[j] );
            swap ( a[i] , a[j] );
        }

    }

    c1 = v[1];

    c2 = v[2];

    i1 = a[1];

    i2 = a[2];

    int rez = c1 + c2 + ( i2 - i1 );

    g << rez;

    return 0;
}