#include<bits/stdc++.h>
using namespace std;
#define REP(i,st,ed) for(register int i=st,i##end=ed;i<=i##end;++i)
#define DREP(i,st,ed) for(register int i=st,i##end=ed;i>=i##end;--i)
typedef long long ll;
inline int read(){
	int x;
	char c;
	int f=1;
	while((c=getchar())!='-' && (c<'0' || c>'9'));
	if(c=='-') c=getchar(),f=-1;
	x=c^'0';
	while((c=getchar())>='0' && c<='9') x=(x<<1)+(x<<3)+(c^'0');
	return x*f;
}
inline ll readll(){
	ll x;
	char c;
	ll f=1;
	while((c=getchar())!='-' && (c<'0' || c>'9'));
	if(c=='-') c=getchar(),f=-1;
	x=c^'0';
	while((c=getchar())>='0' && c<='9') x=(x<<1ll)+(x<<3ll)+(c^'0');
	return x*f;
}
const int maxn=3e5+10,inf=0x3f3f3f3f;
inline bool chkmin(int &x,int y){return (y<x)?(x=y,1):0;}
inline bool chkmax(int &x,int y){return (y>x)?(x=y,1):0;}
int n,w,h;
struct Segment_tree{
	int Max[maxn<<2],tag[maxn<<2];
	void push_down(int x){
		if(tag[x]!=0){
			Max[x<<1]+=tag[x],Max[x<<1|1]+=tag[x];
			tag[x<<1]+=tag[x],tag[x<<1|1]+=tag[x];
			tag[x]=0;
		}
	}
	void build_tree(){
		REP(i,1,n<<2) Max[i]=-inf,tag[i]=0;
	}
	void update(int x,int L,int R,int ql,int qr,int v){
		if(ql<=L && R<=qr){
			tag[x]+=v;Max[x]+=v;
			return;
		}
		int Mid=(L+R)>>1;push_down(x);
		if(ql<=Mid) update(x<<1,L,Mid,ql,qr,v);
		if(qr>Mid) update(x<<1|1,Mid+1,R,ql,qr,v);
		Max[x]=max(Max[x<<1],Max[x<<1|1]);
	}
}Seg;
struct point{
	int x,y;
	bool operator <(const point &rhs) const{
		return x<rhs.x;
	}
}a[maxn];
point sta[maxn],stb[maxn];
int tmpa,tmpb,ans;
void work(){
	Seg.build_tree();
	sort(a+1,a+n+1);
	tmpa=tmpb=0;
	REP(i,1,n-1){
		if(a[i].y<=h/2){
			int lst=i;
			while(tmpa && a[i].y>=sta[tmpa].y){
				Seg.update(1,1,n,sta[tmpa].x,lst-1,sta[tmpa].y-a[i].y);
				lst=sta[tmpa].x;--tmpa;
			}
			if(lst<i) sta[++tmpa]=(point){lst,a[i].y};
		}
		else{
			int lst=i;
			while(tmpb && a[i].y<=stb[tmpb].y){
				Seg.update(1,1,n,stb[tmpb].x,lst-1,a[i].y-stb[tmpb].y);
				lst=stb[tmpb].x,--tmpb;
			}
			if(lst<i) stb[++tmpb]=(point){lst,a[i].y};
		}
		sta[++tmpa]=(point){i,0};
		stb[++tmpb]=(point){i,h};
		Seg.update(1,1,n,i,i,inf+h-a[i].x);
		chkmax(ans,Seg.Max[1]+a[i+1].x);
	}
}
int main(){
	w=read(),h=read(),n=read();
	REP(i,1,n) a[i].x=read(),a[i].y=read();
	a[++n]=(point){0,0},a[++n]=(point){w,h};
	work();
	REP(i,1,n) swap(a[i].x,a[i].y);
	swap(w,h);
	work();
	printf("%d\n",ans*2);
	return 0;
}