#include <iostream>

using namespace std;

int main()
{
    int n,m,lm=0,dm=0,dm1=0,lm1=0,li,di,smax=-1;
    cin>>n>>m;
    for(int i=1;i<=m;i++)
    {
        cin>>di>>li;
        if(lm==0)
            lm=li,dm=di;
        else if(lm1==0)
            lm1=li,dm1=di;
        else if(lm+dm<li+di)
             lm=li,dm=di;
        else if(lm1+dm1<li+di)
            lm1=li,dm1=di;

    }
    if(dm>dm1)
    cout<<lm+lm1+dm-dm1;
    else cout<<lm+lm1+dm1-dm;
    return 0;
}