#define ONLINE_JUDGE
#include<cstdio>
#include<vector>
#include<utility>
#include<algorithm>
#include<cstring>

#define HEAP priority_queue
#define rep(i, n) for(int i = 0, _end_ = (n); i < _end_; ++i)
#define per(i, n) for(int i = (n) - 1; i >= 0 ; --i)
#define forn(i, l, r) for(int i = (l), _end_ = (r); i <= _end_; ++i)
#define nrof(i, r, l) for(int i = (r), _end_ = (l); i >= _end_; --i)
#define FOR(a, b) for(auto (a): (b))
#define mp make_pair
#define mt make_tuple
#define pb push_back
#define X first
#define Y second
#define eps 1e-6
#define pi 3.1415926535897932384626433832795
#define SZ(x) (int)x.size()
#define ALL(x) x.begin(), x.end()
#define FILL(a, b) memset((a), (b), sizeof((a)))
#define MCPY(a, b) memcpy((a), (b), sizeof((b)))

using namespace std;

typedef long long LL;
typedef double flt;
typedef vector<int> vi;
typedef vector<LL> vl;
typedef pair<int,int> pii;
typedef pair<int,LL> pil;
typedef pair<LL,int> pli;
typedef pair<LL,LL> pll;
typedef vector<pil> vil;
typedef vector<pii> vii;
typedef vector<pli> vli;
typedef vector<pll> vll;

const int iinf = 1e9 + 7;
const int oo = 0x3f3f3f3f;
const LL linf = 1ll << 60;
const flt dinf = 1e60;

template <typename T>
inline void scf(T &x)
{
	bool f = 0; x = 0; char c = getchar();
	while((c < '0' || c > '9') && c != '-') c = getchar();
	if(c == '-') { f = 1; c = getchar(); }
	while(c >= '0' && c <= '9') { x = x * 10 + c - '0'; c = getchar(); }
	if(f) x = -x; return;
}
template <typename T1, typename T2>
void scf(T1 &x, T2 &y) { scf(x); return scf(y); }
template <typename T1, typename T2, typename T3>
void scf(T1 &x, T2 &y, T3 &z) { scf(x); scf(y); return scf(z); }
template <typename T1, typename T2, typename T3, typename T4>
void scf(T1 &x, T2 &y, T3 &z, T4 &w) { scf(x); scf(y); scf(z); return scf(w); }

inline char mygetchar(){ char c = getchar(); while(c == ' ' || c == '\n') c = getchar(); return c; }

template <typename T> inline bool chkmax(T &x, const T &y){ return y > x ? x = y, 1 : 0; }
template <typename T> inline bool chkmin(T &x, const T &y){ return y < x ? x = y, 1 : 0; }

#ifdef ONLINE_JUDGE
#define debug(...) ;
#else
#define debug(...) fprintf(stderr, __VA_ARGS__)
#define DEBUG
#endif

//---------------------------------------------------------head----------------------------------------------------

const int maxn = 3e5 + 100;

struct point
{
	int x, y;

	point(int x = 0, int y = 0): x(x), y(y){}
}pnt[maxn];

int M, W, H, n, ans, hsh[maxn], h_n;

namespace seg
{
	struct node
	{
		node *ch[2];
		int num, tag;

		node(int x = -iinf): num(x){ memset(ch, 0, sizeof ch); tag = 0; }

		void push()
		{
			if(tag)
			{
				rep(i, 2) ch[i]->tag += tag, ch[i]->num += tag;
				tag = 0;
			}
			return;
		}

		void pull()
		{
			num = -iinf;
			rep(i, 2) chkmax(num, ch[i]->num);
			return;
		}
	}*root;

	int seg_a, seg_b, seg_q, n;

	void B(node* &u, int l, int r)
	{
		u = new node;
		if(l == r)
		{
			u->num = -hsh[l];
			return;
		}
		int mid = l + r >> 1;
		B(u->ch[0], l, mid);
		B(u->ch[1], mid + 1, r);
		return u->pull();
	}

	void B(int _n)
	{
		root = 0; n = _n;
		B(root, 0, n - 1);
		return;
	}

	void Q(node* &u, int l, int r)
	{
		if(seg_a <= l && r <= seg_b)
		{
			chkmax(seg_q, u->num);
			return;
		}
		int mid = l + r >> 1; u->push();
		if(seg_a <= mid) Q(u->ch[0], l, mid);
		if(mid < seg_b) Q(u->ch[1], mid + 1, r);
		return;
	}

	int Q(int l, int r)
	{
		if(l > r) return -iinf;
		seg_a = l, seg_b = r, seg_q = -iinf;
		Q(root, 0, n - 1);
		return seg_q;
	}

	void M(node* &u, int l, int r)
	{
		if(seg_a <= l && r <= seg_b)
		{
			u->num += seg_q;
			u->tag += seg_q;
			return;
		}
		int mid = l + r >> 1; u->push();
		if(seg_a <= mid) M(u->ch[0], l, mid);
		if(mid < seg_b) M(u->ch[1], mid + 1, r);
		return u->pull();
	}

	void M(int l, int r, int x)
	{
		if(l > r) return;
		//debug("M(%d, %d, %d)\n", l, r, x);
		seg_a = l, seg_b = r, seg_q = x;
		return M(root, 0, n - 1);
	}
}

struct stk
{
	struct item
	{
		int x, y, i;

		item(int x = 0, int y = 0, int i = 0): x(x), y(y), i(i){}
	}num[maxn];

	int n;

	stk(){ n = 0; }

	void push(int x, int y)
	{
		int i = lower_bound(hsh, hsh + h_n, y) - hsh;
		//debug("push(%d, %d)\t%d\n", x, y, i);
		if(!n)
		{
			num[n++] = item(x, y, i);
			return;
		}
		seg::M(num[n - 1].i, i - 1, x);
		while(n && num[n - 1].x > x)
		{
			--n;
			seg::M(n ? num[n - 1].i : 0, num[n].i - 1, x - num[n].x);
		}
		num[n++] = item(x, y, i);
		return;
	}

	void upd(int i, int x)
	{
		int j = n ? num[n - 1].i : 0;
		debug("\tupd(%d %d, %d)\n", j, i, x);
		seg::M(j, i, x);
		return;
	}

	void undo(int i, int x)
	{
		int j = n ? num[n - 1].i : 0;
		seg::M(j, i, -x);
		return;
	}
}*A, *B;

void work()
{
	M = W >> 1; h_n = 0;
	sort(pnt, pnt + n, [&](point a, point b){ return a.y < b.y; });
	rep(i, n) hsh[h_n++] = pnt[i].y;
	h_n = unique(hsh, hsh + h_n) - hsh;
#ifdef DEBUG
	rep(i, n) printf("%d %d\n", pnt[i].x, pnt[i].y);
	debug("%d\n", M);
	rep(i, h_n) printf("%d ", hsh[i]); putchar('\n');
#endif
	seg::B(h_n);
	A = new stk; B = new stk;
	for(int i = -1, j = 0, y = hsh[j]; j < h_n ; y = hsh[++j])
	{
		debug("===============================================================\n%d\n", y);
		while(i + 1 < n && pnt[i + 1].y == y)
		{
			++i;
			if(pnt[i].x <= M) A->push(-pnt[i].x, pnt[i].y);
			else B->push(pnt[i].x, pnt[i].y);
		}
		A->upd(min(j + 1, h_n - 1), 0); B->upd(min(j + 1, h_n - 1), W);
		chkmax(ans, seg::Q(0, j) + ((j == h_n - 1) ? H : hsh[j + 1]));
		A->undo(min(j + 1, h_n - 1), 0); B->undo(min(j + 1, h_n - 1), W);
		debug("%d\n", ans);
	}
	return;
}

int main()
{
	scf(W, H, n);
	for(int i = 0; i < n; ++i)
	{
		int x, y; scf(x, y);
		if(!x || !y){ --i, --n; continue; }
		pnt[i] = point(x, y);
	}
	pnt[n++] = point(0, 0);
	pnt[n++] = point(W, 0);
	pnt[n++] = point(0, H);
	pnt[n++] = point(W, H);
	work();
	swap(W, H);
	rep(i, n) swap(pnt[i].x, pnt[i].y);
	work();
	printf("%d\n", ans << 1);
	return 0;
}

Measured memory_bytes: 55296