#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#define maxn 300010
using namespace std;
int read()
{
    int x=0,f=1;
    char ch=getchar();
    while   (ch-'0'<0||ch-'0'>9){if(ch=='-') f=-1;ch=getchar();}
    while(ch-'0'>=0&&ch-'0'<=9){x=x*10+ch-'0';ch=getchar();}
    return x*f;
}
int w,h,n;
struct P{
    int x,y;
}p[maxn];
bool cmp(P a,P b)
{
    return a.x<b.x;
}
int top1,top2;
struct Segment_Tree{
    int t[maxn*4];
    int tag[maxn*4];
    void build(int k,int l,int r)
    {
        t[k]=tag[k]=0;
        if(l==r)  return;
        int mid=(l+r)/2;
        build(k*2,l,mid);
        build(k*2+1,mid+1,r);
    }
    void update(int k)
    {
        t[k]=max(t[k*2],t[k*2+1]);
    }
    void pushdown(int k)
    {
        if(!tag[k])  return;
        t[k*2]+=tag[k];tag[k*2]+=tag[k];
        t[k*2+1]+=tag[k];tag[k*2+1]+=tag[k];
        tag[k]=0;
    }
    void modi(int k,int l,int r,int x,int y,int z)
    {
        if(l>=x&&r<=y)
        {
            t[k]+=z;
            tag[k]+=z;
            return;
        }
        pushdown(k);
        int mid=(l+r)/2;
        if(mid>=x) modi(k*2,l,mid,x,y,z);
        if(mid<y)  modi(k*2+1,mid+1,r,x,y,z);
        update(k);
    }
}S;
struct X{
    int id,val;
}s1[maxn],s2[maxn];
int ans;
void solve()
{
    sort(p+1,p+n+1,cmp);
    S.build(1,1,n);
    top1=top2=0;
    for(int i=1;i<=n;i++)
    {
        int las=i-1;
        if(p[i].y<=h/2)
        {
            while(top1&&s1[top1].val<p[i].y)
            {
                S.modi(1,1,n,s1[top1].id,las,s1[top1].val-p[i].y);
                las=s1[top1].id-1;
                top1--;
            }
            if(las!=i-1)  s1[++top1]=(X){las+1,p[i].y};
        }
        else{
            while(top2&&s2[top2].val>p[i].y)
            {
                S.modi(1,1,n,s2[top2].id,las,p[i].y-s2[top2].val);
                las=s2[top2].id-1;
                top2--;
            }
            if(las!=i-1)  s2[++top2]=(X){las+1,p[i].y};
        }
        s1[++top1]=(X){i,0};
        s2[++top2]=(X){i,h};
        S.modi(1,1,n,i,i,h-p[i].x);
        ans=max(ans,S.t[1]+p[i+1].x);
    }
}
int main()
{
    w=read();h=read();n=read();
    for(int i=1;i<=n;i++)  p[i].x=read(),p[i].y=read();
    p[++n]=(P){0,0};p[++n]=(P){w,h};
    solve();
    for(int i=1;i<=n;i++)  swap(p[i].x,p[i].y);
    swap(w,h);
    solve();
    printf("%d\n",ans*2);
    return 0;
}