// Slower C++ solution:
#include<algorithm>

#include<cstring>

#include<iostream>

#include<cstdio>

#include<cstdlib>

#include<stack>

using namespace std;



const int maxn = 300010,inf = 1<<29;

int W,H,N,ans,tree[maxn*4],tag[maxn*4];



inline int gi()

{

	char ch; int ret = 0,f = 1;

	do ch = getchar(); while (!(ch >= '0'&&ch <= '9')&&ch != '-');

	if (ch == '-') f = -1,ch = getchar();

	do ret = ret*10+ch-'0',ch = getchar(); while (ch >= '0'&&ch <= '9');

	return ret*f;

}



struct node

{

	int x,y;

	inline node(int _x = 0,int _y = 0):x(_x),y(_y) {}

	inline void read() { x = gi(); y = gi(); }

	friend inline bool operator <(const node &a,const node &b) { return a.x < b.x; }

}P[maxn];



inline void build(int now,int l,int r)

{

	tree[now] = -inf; tag[now] = 0; if (l == r) return;

	int mid = (l+r) >> 1;

	build(now<<1,l,mid); build(now<<1|1,mid+1,r);

}



inline void pushdown(int now,int l,int r)

{

	if (l != r)

	{

		tree[now<<1] += tag[now]; tag[now<<1] += tag[now];

		tree[now<<1|1] += tag[now]; tag[now<<1|1] += tag[now];

	}

	tag[now] = 0;

}



inline void modify(int now,int l,int r,int ql,int qr,int key)

{

	if (tag[now]) pushdown(now,l,r);

	if (l == ql&&r == qr) { tree[now] += key; tag[now] = key; return; }

	int mid = (l+r) >> 1;

	if (qr <= mid) modify(now<<1,l,mid,ql,qr,key);

	else if (ql > mid) modify(now<<1|1,mid+1,r,ql,qr,key);

	else modify(now<<1,l,mid,ql,mid,key),modify(now<<1|1,mid+1,r,mid+1,qr,key);

	tree[now] = max(tree[now<<1],tree[now<<1|1]);

}



inline void change(int now,int l,int r,int pos,int key)

{

	if (tag[now]) pushdown(now,l,r);

	if (l == r) { tree[now] = key; return; }

	int mid = (l+r) >> 1;

	if (pos <= mid) change(now<<1,l,mid,pos,key);

	else change(now<<1|1,mid+1,r,pos,key);

	tree[now] = max(tree[now<<1],tree[now<<1|1]);

}



inline void work()

{

	sort(P+1,P+N+1); build(1,1,N+1);

	stack <node> S1,S2; int mid = H>>1;

	change(1,1,N+1,1,H); S1.push(node(mid,0)); S2.push(node(H-mid,0));

	for (int i = 1;i <= N;++i)

	{

		ans = max(ans,(P[i].x+tree[1])<<1); int h,last;

		if (P[i].y <= mid)

		{

			for (h = mid,last = i;!S1.empty()&&mid-P[i].y < (S1.top()).x;h = (S1.top()).x,last = (S1.top()).y,S1.pop())

				modify(1,1,N+1,(S1.top()).y+1,last,mid-P[i].y-h);

			if (S1.empty()) S1.push(node(mid-P[i].y,0));

			else modify(1,1,N+1,(S1.top()).y+1,last,mid-P[i].y-h);

			S1.push(node(mid-P[i].y,i));

		}

		else

		{

			for (h = H-mid,last = i;!S2.empty()&&P[i].y-mid < (S2.top()).x;h = (S2.top()).x,last = (S2.top()).y,S2.pop())

				modify(1,1,N+1,(S2.top()).y+1,last,P[i].y-mid-h);

			if (S2.empty()) S2.push(node(P[i].y-mid,0));

			else modify(1,1,N+1,(S2.top()).y+1,last,P[i].y-mid-h);

			S2.push(node(P[i].y-mid,i));

		}

		change(1,1,N+1,i+1,H-P[i].x);

	}

	ans = max(ans,(W+tree[1])<<1);

}



int main()

{

//	freopen("D.in","r",stdin);

//	freopen("D.out","w",stdout);

	W = gi(); H = gi(); N = gi();

	for (int i = 1;i <= N;++i) P[i].read();

	ans = max((H+1)<<1,(W+1)<<1);

	work();

	for (int i = 1;i <= N;++i) swap(P[i].x,P[i].y); swap(H,W);

	work();

	cout << ans << endl;

	return 0;

}


// Faster C++ solution for the same problem (measured speedup: 3.2289715490549655x):
#include <iostream>

#include <cstdio>

#include <algorithm>

#define N 300002

using namespace std;

struct SegmentTree{

	int dat,add;

}t[N*4];

struct point{

	int x,y;

}p[N];

int w,h,n,i,ans;

void build(int p,int l,int r)

{

	t[p].dat=t[p].add=0;

	if(l==r) return;

	int mid=(l+r)/2;

	build(p*2,l,mid);

	build(p*2+1,mid+1,r);

}

void update(int p)

{

	t[p].dat=max(t[p*2].dat,t[p*2+1].dat);

}

void spread(int p)

{

	if(t[p].add){

		int tmp=t[p].add;

		t[p].add=0;

		t[p*2].dat+=tmp;t[p*2].add+=tmp;

		t[p*2+1].dat+=tmp;t[p*2+1].add+=tmp;

	}

}

void change(int p,int l,int r,int ql,int qr,int x)

{

	if(ql<=l&&r<=qr){

		t[p].dat+=x;

		t[p].add+=x;

		return;

	}

	int mid=(l+r)/2;

	spread(p);

	if(ql<=mid) change(p*2,l,mid,ql,qr,x);

	if(qr>mid) change(p*2+1,mid+1,r,ql,qr,x);

	update(p);

}

int my_comp(const point &a,const point &b)

{

	return a.x<b.x;

}

void solve()

{

	sort(p+1,p+n+1,my_comp);

	int s1[N],s2[N],top1=0,top2=0;

	s1[0]=s2[0]=1;

	build(1,1,n);

	change(1,1,n,1,n,h*2);

	for(int i=2;i<=n;i++){

		change(1,1,n,1,i-1,(p[i].x-p[i-1].x)*2);

		ans=max(ans,t[1].dat);

		if(p[i].y>=h/2){

			change(1,1,n,s1[top1],i-1,-(h-p[i].y)*2);

			while(top1&&p[s1[top1]].y>=p[i].y){

				change(1,1,n,s1[top1-1],s1[top1]-1,-(p[s1[top1]].y-p[i].y)*2);

				top1--;

			}

			s1[++top1]=i;

		}

		else{

			change(1,1,n,s2[top2],i-1,-p[i].y*2);

			while(top2&&p[s2[top2]].y<=p[i].y){

				change(1,1,n,s2[top2-1],s2[top2]-1,-(p[i].y-p[s2[top2]].y)*2);

				top2--;

			}

			s2[++top2]=i;

		}

	}

}

int main()

{

	cin>>w>>h>>n;

	for(i=2;i<=n+1;i++) cin>>p[i].x>>p[i].y;

	n+=2;

	p[n].x=w;p[n].y=h;

	solve();

	swap(w,h);

	for(i=1;i<=n;i++) swap(p[i].x,p[i].y);

	solve();

	cout<<ans<<endl;

	return 0;

}