#include<bits/stdc++.h>

#define LL long long
#define RG register

using namespace std;
template<class T> inline void read(T &x) {
	x = 0; RG char c = getchar(); bool f = 0;
	while (c != '-' && (c < '0' || c > '9')) c = getchar(); if (c == '-') c = getchar(), f = 1;
	while (c >= '0' && c <= '9') x = x*10+c-48, c = getchar();
	x = f ? -x : x;
	return ;
}
template<class T> inline void write(T x) {
	if (!x) {putchar(48);return ;}
	if (x < 0) x = -x, putchar('-');
	int len = -1, z[20]; while (x > 0) z[++len] = x%10, x /= 10;
	for (RG int i = len; i >= 0; i--) putchar(z[i]+48);return ;
}
const int N = 3e5 + 10;
struct node {
	int x, y;
	bool operator < (const node &z) const {
		return x < z.x;
	}
} a[N];
#define lson (o << 1)
#define rson (o << 1 | 1)
int T[N << 2], P[N << 2], n, W, H;
void pushdown(int o, int l, int r) {
	P[lson] += P[o]; P[rson] += P[o];
	T[lson] += P[o]; T[rson] += P[o];
	P[o] = 0;
}
void pushup(int o) { T[o] = max(T[lson], T[rson]); }
void Modify(int o, int l, int r, int L, int R, int k) {
	if (L <= l && r <= R) { P[o] += k, T[o] += k; return ; }
	pushdown(o, l, r); int mid = (l + r) >> 1;
	if (L <= mid) Modify(lson, l, mid, L, R, k);
	if (R > mid) Modify(rson, mid + 1, r, L, R, k);
	pushup(o);
}
int ans, Q1[N], Q2[N], top1, top2;
void solve() {
	sort(a + 2, a + n + 1);
	memset(T, 0, sizeof(T));
	memset(P, 0, sizeof(P));
	Modify(1, 1, n, 1, n, H);
	top1 = top2 = 0;
	Q1[top1] = Q2[top2] = 1;
	int h = H >> 1;
	for (int i = 2; i <= n; i++) {
		Modify(1, 1, n, 1, i - 1, a[i].x - a[i - 1].x);
		ans = max(ans, T[1]);
		if (a[i].y >= h) {
			Modify(1, 1, n, Q1[top1], i - 1, a[i].y - H);
			while (top1 && a[Q1[top1]].y > a[i].y) {
				Modify(1, 1, n, Q1[top1 - 1], Q1[top1] - 1, a[i].y - a[Q1[top1]].y);
				top1--;
			}
			Q1[++top1] = i;
		}
		else {
			Modify(1, 1, n, Q2[top2], i - 1, -a[i].y);
			while (top2 && a[Q2[top2]].y < a[i].y) {
				Modify(1, 1, n, Q2[top2 - 1], Q2[top2] - 1, a[Q2[top2]].y - a[i].y);
				top2--;
			}
			Q2[++top2] = i;
		}
	}
	return ;
}
int main() {
	//freopen(".in", "r", stdin);
	//freopen(".out", "w", stdout);
	read(W), read(H), read(n); n++;
	for (int i = 2; i <= n; i++) read(a[i].x), read(a[i].y);
	a[++n] = (node) {W, H};
	solve();
	swap(W, H); for (int i = 1; i <= n; i++) swap(a[i].x, a[i].y);
	solve();
	printf("%d\n", ans << 1);
	return 0;
}
