#include <iostream>
#include <cstdio>
#define MAXM 1000050

using namespace std;

int m, n, a[MAXM];
int sol, best;
inline int iabs(int x) { return x < 0 ? -x : x; }


int main()
{
    freopen("orase.in", "r", stdin);
    freopen("orase.out", "w", stdout);

    scanf("%d %d", &m, &n);
    for (int i = 1; i <= n; i++)
    {
        int d, L;
        scanf("%d %d", &d, &L);
        sol = max(sol, a[d]+L);
        a[d] = max(a[d], L);
    }
    int i;
    for (i = 0; a[i] == 0; i++);
    for (i; i <= m; i++) {
        if (a[i])
            sol = max(sol, best+1+a[i]);
        best = max(best+1, a[i]);
    }
    printf("%d", sol);

    return 0;
}