#include<bits/stdc++.h>
using namespace std;
ifstream in("orase.in");
ofstream out("orase.out");
int n,m,i,D,L,max1[1000005],max2[1000005];
int main(){
    in>>m>>n;
    for(;i<=m;++i)
        max1[i]=max2[i]=-0x3f3f3f3f;
    for(i=0;i<n;++i)
    {
        in>>D>>L;
        if(L>max1[D])
            max2[D]=max1[D],
            max1[D]=L;
        else
            if(L>max2[D])
                max2[D]=L;
    }
    int maxx=-0x3f3f3f3f,max_c=-0x3f3f3f3f;
    for(i=0;i<=m;++i)
    {
        if(maxx < max1[i]+max2[i])
            maxx = max1[i]+max2[i];
        max_c++;
        if(maxx < max_c+max1[i])
            maxx = max_c+max1[i];
        if(max_c <
            max1[i])
            max_c = max1[i];
    }
    out<<maxx;
    return 0;
}