#include <bits/stdc++.h>

using namespace std;

const int maxn = 300010;
int w,h,n,tag[maxn << 2],maxx[maxn << 2],ans,top1,top2;

struct node
{
	int x,y;
}e[maxn];

struct node2
{
	int x,y;
}S1[maxn],S2[maxn];

bool cmp(node a,node b)
{
	if (a.x == b.x)
		return a.y < b.y;
	return a.x < b.x;
}

void pushup(int o)
{
	maxx[o] = max(maxx[o * 2],maxx[o * 2 + 1]);
}

void pushdown(int o)
{
	if (tag[o])
	{
		tag[o * 2] += tag[o];
		tag[o * 2 + 1] += tag[o];
		maxx[o * 2] += tag[o];
		maxx[o * 2 + 1] += tag[o];
		tag[o] = 0;
	}
}

void build(int o,int l,int r)
{
	tag[o] = maxx[o] = 0;
	if (l == r)
		return;
	int mid = (l + r) >> 1;
	build(o * 2,l,mid);
	build(o * 2 + 1,mid + 1,r);
}

void update(int o,int l,int r,int x,int y,int v)
{
	if (x <= l && r <= y)
	{
		tag[o] += v;
		maxx[o] += v;
		return;
	}
	pushdown(o);
	int mid = (l + r) >> 1;
	if (x <= mid)
		update(o * 2,l,mid,x,y,v);
	if (y > mid)
		update(o * 2 + 1,mid + 1,r,x,y,v);
	pushup(o);
}

void solve()
{
	for (int i = 0; i <= top1; i++)
		S1[i].x = S1[i].y = 0;
	top1 = 0;
	for (int i = 0; i <= top2; i++)
		S2[i].x = S2[i].y = 0;
	top2 = 0;
	S1[0].y = S2[0].y = 1;
	sort(e + 2,e + 1 + n,cmp);
	build(1,1,n);
	update(1,1,n,1,n,h << 1);
	for (int i = 2; i <= n; i++)
	{
		update(1,1,n,1,i - 1,(e[i].x - e[i - 1].x) << 1);
		ans = max(ans,maxx[1]);
		if (e[i].y <= h / 2)
		{
			update(1,1,n,S1[top1].y,i - 1,-e[i].y * 2);
			while (top1 && S1[top1].x <= e[i].y)
			{
				update(1,1,n,S1[top1 - 1].y,S1[top1].y - 1,(S1[top1].x - e[i].y) * 2);
				top1--;
			}
			++top1;
			S1[top1].x = e[i].y;
			S1[top1].y = i;
		}
		else
		{
			update(1,1,n,S2[top2].y,i - 1,(e[i].y - h) * 2);
			while (top2 && e[i].y <= S2[top2].x)
			{
				update(1,1,n,S2[top2 - 1].y,S2[top2].y - 1,(e[i].y - S2[top2].x) * 2);
				top2--;
			}
			++top2;
			S2[top2].x = e[i].y;
			S2[top2].y = i;
		}
	}
}

int main()
{
	scanf("%d%d%d",&w,&h,&n);
	n++;
	for (int i = 2; i <= n; i++)
		scanf("%d%d",&e[i].x,&e[i].y);
	e[++n].x = w;
	e[n].y = h;
	solve();
	swap(w,h);
	for (int i = 1; i <= n; i++)
		swap(e[i].x,e[i].y);
	solve();
	printf("%d\n",ans);

	return 0;
}
