#include <bits/stdc++.h>

#define x first
#define y second

typedef std::pair<int, int> point;
const int N = 600005;

int totx, toty, vx[N], vy[N], ans;
int w, h, n;

point p[N];

int tag[N << 2], max[N << 2];

void pushup(int o) { max[o] = std::max(max[o << 1], max[o << 1 | 1]) + tag[o]; }
void build(int o = 1, int l = 0, int r = toty - 1) {
	tag[o] = 0;
	if (l == r) { max[o] = -vy[l] + w; return; }
	int mid = l + r >> 1;
	build(o << 1, l, mid), build(o << 1 | 1, mid + 1, r), pushup(o);
}
void modify(int L, int R, int v, int o = 1, int l = 0, int r = toty - 1) {
	if (L > R) return;
	if (L <= l && R >= r) { tag[o] += v, max[o] += v; return; }
	int mid = l + r >> 1;
	if (L <= mid) modify(L, R, v, o << 1, l, mid);
	if (R > mid) modify(L, R, v, o << 1 | 1, mid + 1, r);
	pushup(o);
}

void solve() {
	int mid = std::upper_bound(vx, vx + totx, w >> 1) - vx - 1;
	build();
	std::sort(p + 1, p + n + 1, [&] (point a, point b) { return a.y != b.y ? a.y < b.y : std::abs(a.x - mid) > std::abs(b.x - mid); });
	std::stack<int> L, R;
	p[0] = point(mid, 0), L.emplace(0), R.emplace(0);
	for (int y = 0, i = 0; y < toty; ++y) {
		ans = std::max(ans, vy[y] + max[1]);
		for (; i <= n && p[i].y == y; ++i)
			if (p[i].x <= mid) {
				modify(p[L.top()].y, y - 1, -vx[p[i].x]);
				while (p[L.top()].x < p[i].x) {
					point lst = p[L.top()]; L.pop();	
					modify(p[L.top()].y, lst.y - 1, vx[lst.x] - vx[p[i].x]);
				}
				L.emplace(i);
			} else {
				modify(p[R.top()].y, y - 1, vx[p[i].x] - w);
				while (p[R.top()].x > p[i].x) {
					point lst = p[R.top()]; R.pop();
					modify(p[R.top()].y, lst.y - 1, vx[p[i].x] - vx[lst.x]);
				}
				R.emplace(i);
			}
	}
}

int main() {
	std::ios::sync_with_stdio(0), std::cin.tie(0);
	std::cin >> w >> h >> n, ans = std::max(w, h) + 1;
	vx[0] = vy[0] = 0;
	for (int i = 1; i <= n; ++i) {
		std::cin >> p[i].x >> p[i].y; 
		if (p[i].x == 0 || p[i].x == w || p[i].y == 0 || p[i].y == h)
			{--i, --n; continue; }
		vx[i] = p[i].x, vy[i] = p[i].y;
	}
	vx[n + 1] = w, vy[n + 1] = h;
	totx = toty = n + 2;
	totx = (std::sort(vx, vx + totx), std::unique(vx, vx + totx) - vx);
	toty = (std::sort(vy, vy + toty), std::unique(vy, vy + toty) - vy);
	for (int i = 1; i <= n; ++i)
		p[i].x = std::lower_bound(vx, vx + totx, p[i].x) - vx, p[i].y = std::lower_bound(vy, vy + toty, p[i].y) - vy;
	solve();
	std::swap(w, h), std::swap(totx, toty), std::swap(vx, vy);
	for (int i = 1; i <= n; ++i) std::swap(p[i].x, p[i].y);
	solve();
	std::cout << ans * 2 << '\n';
	return 0;
}