#include <bits/stdc++.h>
using namespace std;

using ll=long long;
#define int ll

#define rng(i,a,b) for(int i=int(a);i<int(b);i++)
#define rep(i,b) rng(i,0,b)
#define gnr(i,a,b) for(int i=int(b)-1;i>=int(a);i--)
#define per(i,b) gnr(i,0,b)
#define pb push_back
#define eb emplace_back
#define a first
#define b second
#define bg begin()
#define ed end()
#define all(x) x.bg,x.ed
#ifdef LOCAL
#define dmp(x) cerr<<__LINE__<<" "<<#x<<" "<<x<<endl
#else
#define dmp(x) void(0)
#endif

template<class t,class u> void chmax(t&a,u b){if(a<b)a=b;}
template<class t,class u> void chmin(t&a,u b){if(b<a)a=b;}

template<class t> using vc=vector<t>;
template<class t> using vvc=vc<vc<t>>;

using pi=pair<int,int>;
using vi=vc<int>;

template<class t,class u>
ostream& operator<<(ostream& os,const pair<t,u>& p){
	return os<<"{"<<p.a<<","<<p.b<<"}";
}

template<class t> ostream& operator<<(ostream& os,const vc<t>& v){
	os<<"{";
	for(auto e:v)os<<e<<",";
	return os<<"}";
}

#define mp make_pair
#define mt make_tuple
#define one(x) memset(x,-1,sizeof(x))
#define zero(x) memset(x,0,sizeof(x))
#ifdef LOCAL
void dmpr(ostream&os){os<<endl;}
template<class T,class... Args>
void dmpr(ostream&os,const T&t,const Args&... args){
	os<<t<<" ";
	dmpr(os,args...);
}
#define dmp2(...) dmpr(cerr,"Line:",__LINE__,##__VA_ARGS__)
#else
#define dmp2(...) void(0)
#endif

using uint=unsigned;
using ull=unsigned long long;

template<class t,size_t n>
ostream& operator<<(ostream&os,const array<t,n>&a){
	return os<<vc<t>(all(a));
}

template<int i,class T>
void print_tuple(ostream&,const T&){
}

template<int i,class T,class H,class ...Args>
void print_tuple(ostream&os,const T&t){
	if(i)os<<",";
	os<<get<i>(t);
	print_tuple<i+1,T,Args...>(os,t);
}

template<class ...Args>
ostream& operator<<(ostream&os,const tuple<Args...>&t){
	os<<"{";
	print_tuple<0,tuple<Args...>,Args...>(os,t);
	return os<<"}";
}

void print(ll x,int suc=1){
	cout<<x;
	if(suc==1)
		cout<<"\n";
	if(suc==2)
		cout<<" ";
}

ll read(){
	ll i;
	cin>>i;
	return i;
}

vi readvi(int n,int off=0){
	vi v(n);
	rep(i,n)v[i]=read()+off;
	return v;
}

template<class T>
void print(const vector<T>&v,int suc=1){
	rep(i,v.size())
		print(v[i],i==int(v.size())-1?suc:2);
}

string readString(){
	string s;
	cin>>s;
	return s;
}

template<class T>
T sq(const T& t){
	return t*t;
}

//#define CAPITAL
void yes(bool ex=true){
	#ifdef CAPITAL
	cout<<"YES"<<endl;
	#else
	cout<<"Yes"<<endl;
	#endif
	if(ex)exit(0);
}
void no(bool ex=true){
	#ifdef CAPITAL
	cout<<"NO"<<endl;
	#else
	cout<<"No"<<endl;
	#endif
	if(ex)exit(0);
}
void possible(bool ex=true){
	#ifdef CAPITAL
	cout<<"POSSIBLE"<<endl;
	#else
	cout<<"Possible"<<endl;
	#endif
	if(ex)exit(0);
}
void impossible(bool ex=true){
	#ifdef CAPITAL
	cout<<"IMPOSSIBLE"<<endl;
	#else
	cout<<"Impossible"<<endl;
	#endif
	if(ex)exit(0);
}

constexpr ll ten(int n){
	return n==0?1:ten(n-1)*10;
}

const ll infLL=LLONG_MAX/3;

#ifdef int
const int inf=infLL;
#else
const int inf=INT_MAX/2-100;
#endif

int topbit(signed t){
	return t==0?-1:31-__builtin_clz(t);
}
int topbit(ll t){
	return t==0?-1:63-__builtin_clzll(t);
}
int botbit(signed a){
	return a==0?32:__builtin_ctz(a);
}
int botbit(ll a){
	return a==0?64:__builtin_ctzll(a);
}
int popcount(signed t){
	return __builtin_popcount(t);
}
int popcount(ll t){
	return __builtin_popcountll(t);
}
bool ispow2(int i){
	return i&&(i&-i)==i;
}
int mask(int i){
	return (int(1)<<i)-1;
}

bool inc(int a,int b,int c){
	return a<=b&&b<=c;
}

template<class t> void mkuni(vc<t>&v){
	sort(all(v));
	v.erase(unique(all(v)),v.ed);
}

ll rand_int(ll l, ll r) { //[l, r]
	#ifdef LOCAL
	static mt19937_64 gen;
	#else
    static random_device rd;
    static mt19937_64 gen(rd());
    #endif
    return uniform_int_distribution<ll>(l, r)(gen);
}

template<class t>
int lwb(const vc<t>&v,const t&a){
	return lower_bound(all(v),a)-v.bg;
}

signed main(){
	cin.tie(0);
	ios::sync_with_stdio(0);
	cout<<fixed<<setprecision(20);
	
	int wid,hei;cin>>wid>>hei;
	wid--;hei--;
	int n;cin>>n;
	bool dbg=n<0;
	if(dbg){
		n=-n;
	}
	vc<pi> xy;
	rep(i,n){
		int x,y;
		if(dbg){
			static set<pi> tmp;
			do{
				x=rand_int(0,wid);
				y=rand_int(0,hei);
			}while(tmp.count(pi(x,y)));
			tmp.emplace(x,y);
		}else{
			cin>>x>>y;
			x--;y--;
		}
		xy.eb(x,y);
	}
	int ans=0;
	{
		int a=inf,b=-inf,c=inf,d=-inf;
		for(auto p:xy){
			int x,y;tie(x,y)=p;
			chmin(a,x+y);
			chmax(b,x+y);
			chmin(c,x-y);
			chmax(d,x-y);
		}
		if(a!=b&&c!=d&&b-a==d-c&&(a+b)%2==0){
			bool z=true;
			for(auto p:xy){
				int x,y;tie(x,y)=p;
				bool ok=false;
				if(a==x+y)
					ok=1;
				if(b==x+y)
					ok=1;
				if(c==x-y)
					ok=1;
				if(d==x-y)
					ok=1;
				z&=ok;
			}
			if(z)
				ans--;
		}
	}
	dmp(ans);
	rep(_,2){
		dmp(xy);
		int a=inf,b=-inf,c=inf,d=-inf;
		for(auto p:xy){
			int x,y;tie(x,y)=p;
			chmin(a,x+y);
			chmax(b,x+y);
			chmin(c,x-y);
			chmax(d,x-y);
		}
		if(a==b)goto relka;
		if((a+b)%2)goto relka;
		if(d-c>b-a)goto relka;
		{
				
			vi z;
			for(auto p:xy){
				int x,y;tie(x,y)=p;
				if(a!=x+y&&b!=x+y)
					z.pb(x-y);
			}
			
			mkuni(z);
			dmp(z);
			if(z.size()){
				bool ok=true;
				for(auto v:z){
					if(v!=c&&v!=d)
						ok=false;
				}
				if(ok){
					if(z.size()==1){
						if((a+z[0])%2==0){
							if(c==d)
								ans++;
							ans++;
						}
					}else if(d-c==b-a){
						if((a+c)%2==0&&(a+d)%2==0)
							ans++;
					}
				}
			}else{
				int e=c,f=d;
				if((a+c)%2)e--;
				if((a+d)%2)f++;
				dmp2(e,f);
				ans+=max(((b-a)-(f-e))/2+1,int(0));
			}
			
		}
		relka:;
		
		dmp(ans);
		
		for(auto&p:xy)
			p=pi(wid-p.a,p.b);
	}
	rep(_,4){
		int a=inf,b=-inf,c=inf,d=-inf;
		for(auto p:xy){
			int x,y;tie(x,y)=p;
			chmin(a,x+y);
			chmax(b,x+y);
			chmin(c,x-y);
			chmax(d,x-y);
		}
		if((a+c)%2==0){
		
			bool ng=false;
			for(auto p:xy){
				int x,y;tie(x,y)=p;
				if(a!=x+y&&c!=x-y)
					ng=true;
			}
			if(!ng){
				int mx=-inf;
				for(auto p:xy){
					int x,y;tie(x,y)=p;
					chmax(mx,x);
				}
				ans+=wid-mx;
			}
		}
		
		dmp(xy);
		dmp(ans);
		for(auto&p:xy)
			p=pi(hei-p.b,p.a);
		swap(hei,wid);
	}
	rep(_,4){
		int a=inf,b=-inf,c=inf,d=-inf;
		for(auto p:xy){
			int x,y;tie(x,y)=p;
			chmin(a,x+y);
			chmax(b,x+y);
			chmin(c,x-y);
			chmax(d,x-y);
		}
		if(a==b){
			int e=0,f=0;
			for(auto p:xy){
				int x,y;tie(x,y)=p;
				chmax(e,y);
				chmax(f,x);
			}
			ans+=(hei-e)*(wid-f);
		}
		
		dmp(xy);
		dmp(ans);
		for(auto&p:xy)
			p=pi(hei-p.b,p.a);
		swap(hei,wid);
	}
	cout<<ans<<endl;
}