#include<bits/stdc++.h>

typedef unsigned int uint;
typedef long long ll;
typedef unsigned long long ull;
typedef double lf;
typedef long double llf;
typedef std::pair<int,int> pii;

#define xx first
#define yy second

template<typename T> inline T max(T a,T b){return a>b?a:b;}
template<typename T> inline T min(T a,T b){return a<b?a:b;}
template<typename T> inline T abs(T a){return a>0?a:-a;}
template<typename T> inline bool repr(T &a,T b){return a<b?a=b,1:0;}
template<typename T> inline bool repl(T &a,T b){return a>b?a=b,1:0;}
template<typename T> inline T gcd(T a,T b){T t;if(a<b){while(a){t=a;a=b%a;b=t;}return b;}else{while(b){t=b;b=a%b;a=t;}return a;}}
template<typename T> inline T sqr(T x){return x*x;}
#define mp(a,b) std::make_pair(a,b)
#define pb push_back
#define I inline
#define mset(a,b) memset(a,b,sizeof(a))
#define mcpy(a,b) memcpy(a,b,sizeof(a))

#define fo0(i,n) for(int i=0,i##end=n;i<i##end;i++)
#define fo1(i,n) for(int i=1,i##end=n;i<=i##end;i++)
#define fo(i,a,b) for(int i=a,i##end=b;i<=i##end;i++)
#define fd0(i,n) for(int i=(n)-1;~i;i--)
#define fd1(i,n) for(int i=n;i;i--)
#define fd(i,a,b) for(int i=a,i##end=b;i>=i##end;i--)
#define foe(i,x)for(__typeof((x).end())i=(x).begin();i!=(x).end();++i)

struct Cg{I char operator()(){return getchar();}};
struct Cp{I void operator()(char x){putchar(x);}};
#define OP operator
#define RT return *this;
#define RX x=0;char t=P();while((t<'0'||t>'9')&&t!='-')t=P();bool f=0;\
if(t=='-')t=P(),f=1;x=t-'0';for(t=P();t>='0'&&t<='9';t=P())x=x*10+t-'0'
#define RL if(t=='.'){lf u=0.1;for(t=P();t>='0'&&t<='9';t=P(),u*=0.1)x+=u*(t-'0');}if(f)x=-x
#define RU x=0;char t=P();while(t<'0'||t>'9')t=P();x=t-'0';for(t=P();t>='0'&&t<='9';t=P())x=x*10+t-'0'
#define TR *this,x;return x;
I bool IS(char x){return x==10||x==13||x==' ';}template<typename T>struct Fr{T P;I Fr&OP,(int&x)
{RX;if(f)x=-x;RT}I OP int(){int x;TR}I Fr&OP,(ll &x){RX;if(f)x=-x;RT}I OP ll(){ll x;TR}I Fr&OP,(char&x)
{for(x=P();IS(x);x=P());RT}I OP char(){char x;TR}I Fr&OP,(char*x){char t=P();for(;IS(t);t=P());if(~t){for(;!IS
(t)&&~t;t=P())*x++=t;}*x++=0;RT}I Fr&OP,(lf&x){RX;RL;RT}I OP lf(){lf x;TR}I Fr&OP,(llf&x){RX;RL;RT}I OP llf()
{llf x;TR}I Fr&OP,(uint&x){RU;RT}I OP uint(){uint x;TR}I Fr&OP,(ull&x){RU;RT}I OP ull(){ull x;TR}};Fr<Cg>in;
#define WI(S) if(x){if(x<0)P('-'),x=-x;char s[S],c=0;while(x)s[c++]=x%10+'0',x/=10;while(c--)P(s[c]);}else P('0')
#define WL if(y){lf t=0.5;for(int i=y;i--;)t*=0.1;if(x>=0)x+=t;else x-=t,P('-');*this,(ll)(abs(x));P('.');if(x<0)\
x=-x;while(y--){x*=10;x-=floor(x*0.1)*10;P(((int)x)%10+'0');}}else if(x>=0)*this,(ll)(x+0.5);else *this,(ll)(x-0.5);
#define WU(S) if(x){char s[S],c=0;while(x)s[c++]=x%10+'0',x/=10;while(c--)P(s[c]);}else P('0')
template<typename T>struct Fw{T P;I Fw&OP,(int x){WI(10);RT}I Fw&OP()(int x){WI(10);RT}I Fw&OP,(uint x){WU(10);RT}
I Fw&OP()(uint x){WU(10);RT}I Fw&OP,(ll x){WI(19);RT}I Fw&OP()(ll x){WI(19);RT}I Fw&OP,(ull x){WU(20);RT}I Fw&OP()
(ull x){WU(20);RT}I Fw&OP,(char x){P(x);RT}I Fw&OP()(char x){P(x);RT}I Fw&OP,(const char*x){while(*x)P(*x++);RT}
I Fw&OP()(const char*x){while(*x)P(*x++);RT}I Fw&OP()(lf x,int y){WL;RT}I Fw&OP()(llf x,int y){WL;RT}};Fw<Cp>out;

const int N=300007;

struct node
{
	int ma,tag;
}f[1<<20];

inline void pu(int x)
{
	f[x].ma=max(f[x<<1].ma,f[x<<1|1].ma)+f[x].tag;
}

inline void modify(int x,int l,int r,int ql,int qr,int v)
{
	if(l>=ql&&r<=qr){f[x].ma+=v,f[x].tag+=v;return;}
	int t=(l+r)>>1;
	if(ql<=t)modify(x<<1,l,t,ql,qr,v);
	if(qr>t)modify(x<<1|1,t+1,r,ql,qr,v);
	pu(x);
}

int n,w,h,vm,ml,mr,v[N],sl[N],sr[N],sv[N];
pii s[N];

inline void build(int x,int l,int r)
{
	if(l==r)f[x].ma=h-v[l-1];
	else
	{
		int t=(l+r)>>1;
		build(x<<1,l,t);
		build(x<<1|1,t+1,r);
		pu(x);
	}
}

inline int sol()
{
	std::sort(s+1,s+n+1);
	s[++n]=mp(w,h);
	s[n+1].yy=h/2;
	sl[ml++]=n+1,sr[mr++]=n+1;
	fo(i,0,n)v[i]=s[i].xx;
	std::sort(v,v+n+1);
	vm=std::unique(v,v+n+1)-v;
	build(1,1,vm);
	//fo0(i,vm)out,v[i],' ';out,'\n';
	sv[n+1]=1;
	int ans=0;
	for(int i=1,j=1;i<vm;i++)
	{
		//out,i,':',f[1].ma,' ',v[i],'\n';
		repr(ans,f[1].ma+v[i]);
		for(;j<=n&&s[j].xx<v[i];j++);
		for(;j<=n&&s[j].xx==v[i];j++)
		{
			//out,"ok\n";
			sv[j]=i+1;
			if(s[j].yy<=h/2&&(s[j+1].yy>h/2||s[j+1].xx!=v[i]||j==n))
			{
				for(;s[j].yy>s[sl[ml-1]].yy;ml--)
					modify(1,1,vm,sv[sl[ml-2]],sv[sl[ml-1]]-1,s[sl[ml-1]].yy);
				modify(1,1,vm,sv[sl[ml-1]],i,-s[j].yy);
				sl[ml++]=j;
			}
			else if(s[j].yy>h/2)
			{
				for(;s[j].yy<s[sr[mr-1]].yy;mr--)
					modify(1,1,vm,sv[sr[mr-2]],sv[sr[mr-1]]-1,h-s[sr[mr-1]].yy);
				modify(1,1,vm,sv[sr[mr-1]],i,s[j].yy-h);
				sr[mr++]=j;
				break;
			}
		}
		for(;j<=n&&s[j].xx==v[i];j++);
	}
	--n;
	vm=ml=mr=0;
	mset(f,0);
	//out,"sol:ans=",ans,'\n';
	return ans*2;
}

int main()
{
	in,w,h,n;
	fo1(i,n)in,s[i].xx,s[i].yy;
	int ans=sol();
	std::swap(w,h);
	fo1(i,n)std::swap(s[i].xx,s[i].yy);
	repr(ans,sol());
	repr(ans,(max(w,h)+1)*2);
	out,ans,'\n';
}

Measured memory_bytes: 15616