#include <cstdio>
#include <cctype>
#include <cstring>
#include <algorithm>
#define repu(i,x,y) for (int i=x; i<=y; ++i)
#define lc x<<1
#define rc x<<1|1
#define mid ((l+r)>>1)
using namespace std;

int w,h,n,ans,stk1[300100],top1,stk2[300100],top2,maxw[1100000],tag[1100000],ql,qr;
struct data
{
    int x,y;
    bool operator<(const data &t) const
    {
        return x<t.x;
    }
} a[300100];

int getint()
{
    char ch;
    while (!isdigit(ch=getchar()));
    int x=ch-'0';
    for (; isdigit(ch=getchar()); x=x*10+ch-'0');
    return x;
}

void update(int x)
{
    maxw[x]=max(maxw[lc],maxw[rc])+tag[x];
}

void build(int x,int l,int r)
{
    if (l==r)
    {
        maxw[x]=-a[l].x;
        return;
    }
    build(lc,l,mid),build(rc,mid+1,r);
    update(x);
}

void modify(int x,int l,int r,int w)
{
    if (ql<=l && qr>=r)
    {
        tag[x]+=w,maxw[x]+=w;
        return;
    }
    if (ql<=mid)
        modify(lc,l,mid,w);
    if (qr>mid)
        modify(rc,mid+1,r,w);
    update(x);
}

void modify(int l,int r,int w)
{
    if (l<=r)
        ql=l,qr=r,modify(1,1,n,w);
}

void solve()
{
    sort(a+1,a+1+n);
    memset(tag,0,sizeof(tag));
    build(1,1,n);
    top1=top2=0,stk1[0]=stk2[0]=1;
    repu(i,1,n)
    {
        ans=max(ans,a[i].x+maxw[1]);
        if (a[i].y<=h/2)
        {
            modify(stk1[top1],i-1,-h/2);
            for (; top1 && a[stk1[top1]].y<a[i].y; --top1)
                modify(stk1[top1-1],stk1[top1]-1,a[stk1[top1]].y-h/2);
            stk1[++top1]=i,modify(stk1[top1-1],i-1,h/2-a[i].y);
        }
        else
        {
            modify(stk2[top2],i-1,h/2-h);
            for (; top2 && a[stk2[top2]].y>a[i].y; --top2)
                modify(stk2[top2-1],stk2[top2]-1,h/2-a[stk2[top2]].y);
            stk2[++top2]=i,modify(stk2[top2-1],i-1,a[i].y-h/2);
        }
        modify(i,i,h);
    }
}

int main()
{
    scanf("%d%d%d",&w,&h,&n);
    repu(i,1,n)
        a[i].x=getint(),a[i].y=getint();
    a[++n]=(data){0,0};
    a[++n]=(data){w,0};
    a[++n]=(data){0,h};
    a[++n]=(data){w,h};
    solve();
    repu(i,1,n)
        swap(a[i].x,a[i].y);
    swap(w,h);
    solve();
    printf("%d\n",ans*2);
    return 0;
}