#include <fstream>
#include <iostream>

#define sFin "orase.in"
#define sFout "orase.out"
#define nMax 1000069
#define abs(x) (x<0?-1*x:x)
#define s(x) (x[0]+x[1])

using namespace std;

int n,m,a[2],b[2];

void mainLoop(){
	ifstream fin(sFin);
	fin >> m >> n;
	a[0] = -1 * int(2e9);
	b[0] = -1 * int(2e9);

	int x, y;
	for (int i=0;i<n;i++){
		fin >> x >> y;
		if ( s(a) <= s(b) && s(a)<x+y)
			a[0] = x, a[1] = y;
		else
			if (s(b)<s(a) && s(b)<x+y)
				b[0] = x, b[1] = y;
	}
	fout << a[1]+b[1]+abs(a[0]-b[0]);
}

int main(){
	mainLoop();
	return 0;
}