#include <cstdio>

const int MAX_SIZE(50000);

int n, best;

struct street
{
	int d;
	int l;
} heap [MAX_SIZE];

inline void swap (int a, int b)
{
	heap[a].d ^= heap[b].d;
	heap[b].d ^= heap[a].d;
	heap[a].d ^= heap[b].d;
	heap[a].l ^= heap[b].l;
	heap[b].l ^= heap[a].l;
	heap[a].l ^= heap[b].l;
}

inline void read (void)
{
	std::freopen("orase.in","r",stdin);
	int m;
	std::scanf("%d%d",&m,&n);
	for (struct street *iterator(heap), *end(heap + n) ; iterator < end ; ++iterator)
		std::scanf("%d%d",&(iterator->d),&(iterator->l));
	std::fclose(stdout);
}

inline void print (void)
{
	std::freopen("orase.out","w",stdout);
	std::printf("%d\n",best);
	std::fclose(stdout);
}

inline void increase (int index, const int LIMIT)
{
	int left((index << 1) + 1), right(left + 1), greatest;
	while (true)
	{
		if (left < LIMIT && heap[left].d > heap[index].d)
			greatest = left;
		else
			greatest = index;
		if (right < LIMIT && heap[right].d > heap[greatest].d)
			greatest = right;
		if (greatest == index)
			break;
		swap(index,greatest);
		index = greatest;
		left = (index << 1) + 1;
		right = left + 1;
	}
}

inline void build_heap (void)
{
	for (int index((n - 2) >> 1) ; index >= 0 ; --index)
		increase(index,n);
}

inline void heap_sort (void)
{
	build_heap();
	int size(n - 1);
	while (true)
	{
		swap(0,size);
		--size;
		if (size)
			increase(0,size);
		else
			break;
	}
}

inline void dynamic (void)
{
	int max(heap->l - heap->d);
	for (struct street *iterator(heap + 1), *end(heap + n) ; iterator < end ; ++iterator)
	{
		if (iterator->d + iterator->l + max > best)
			best = iterator->d + iterator->l + max;
		if (iterator->l - iterator->d > max)
			max = iterator->l - iterator->d;
	}
}

int main (void)
{
	read();
	heap_sort();
	dynamic();
	print();
	return 0;
}