#include <bits/stdc++.h>
using namespace std;
using LL = long long int;
#define incID(i, l, r) for(int i = (l)    ; i <  (r); ++i)
#define incII(i, l, r) for(int i = (l)    ; i <= (r); ++i)
#define decID(i, l, r) for(int i = (r) - 1; i >= (l); --i)
#define decII(i, l, r) for(int i = (r)    ; i >= (l); --i)
#define inc(i, n)  incID(i, 0, n)
#define inc1(i, n) incII(i, 1, n)
#define dec(i, n)  decID(i, 0, n)
#define dec1(i, n) decII(i, 1, n)
#define inID(v, l, r) ((l) <= (v) && (v) <  (r))
#define inII(v, l, r) ((l) <= (v) && (v) <= (r))
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define FI first
#define SE second
#define  ALL(v)  v.begin(),  v.end()
#define RALL(v) v.rbegin(), v.rend()
auto setmin   = [](auto & a, auto b) { return (b <  a ? a = b, true : false); };
auto setmax   = [](auto & a, auto b) { return (b >  a ? a = b, true : false); };
auto setmineq = [](auto & a, auto b) { return (b <= a ? a = b, true : false); };
auto setmaxeq = [](auto & a, auto b) { return (b >= a ? a = b, true : false); };
auto fl = [](auto a, auto b) { assert(b != 0); return a / b - (a % b != 0 && ((a >= 0) != (b >= 0)) ? 1 : 0); };
auto ce = [](auto a, auto b) { assert(b != 0); return a / b + (a % b != 0 && ((a >= 0) == (b >= 0)) ? 1 : 0); };
auto mo = [](auto a, auto b) { assert(b != 0); a %= b; if(a < 0) { a += abs(b); } return a; };
LL gcd(LL a, LL b) { return (b == 0 ? a : gcd(b, a % b)); }
LL lcm(LL a, LL b) { return a / gcd(a, b) * b; }
#define bit(b, i) (((b) >> (i)) & 1)
#define SI(v) static_cast<int>(v.size())
#define RF(e, v) for(auto & e: v)
#define until(e) while(! (e))
#define if_not(e) if(! (e))
#define ef else if
#define UR assert(false)

// ----

#define T0(T) T
#define T1(T) vector<T0(T)>
#define T2(T) vector<T1(T)>
#define V0(v, T     ...) v
#define V1(v, T, n, ...) T1(T)(n, V0(v, T, __VA_ARGS__))
#define V2(v, T, n, ...) T2(T)(n, V1(v, T, __VA_ARGS__))

int main() {
	LL h, w, n;
	cin >> h >> w >> n;
	vector<LL> x(n), y(n);
	inc(i, n) { cin >> x[i] >> y[i]; }
	
	LL INF = 1e12;
	LL a = +INF, A = -INF;
	LL b = +INF, B = -INF;
	set<int> parity;
	inc(i, n) {
		setmin(a, x[i] - y[i]);
		setmax(A, x[i] - y[i]);
		setmin(b, x[i] + y[i]);
		setmax(B, x[i] + y[i]);
		parity.insert(mo(x[i] + y[i], 2));
	}
	if(SI(parity) == 2) { cout << 0 << endl; return 0; }
	
	using VPLL = vector<pair<LL, LL>>;
	auto v = V2(VPLL(), VPLL, 3, 3);
	inc(i, n) {
		int s = 1;
		if(x[i] - y[i] == a) { s = 0; }
		ef(x[i] - y[i] == A) { s = 2; }
		
		int t = 1;
		if(x[i] + y[i] == b) { t = 0; }
		ef(x[i] + y[i] == B) { t = 2; }
		
		if(SI(v[s][t]) == 0) { v[s][t].EB(x[i], y[i]); };
	}
	
	set<pair<LL, LL>> done;
	auto ok = [&](LL X, LL Y) -> bool {
		bool exist = ! done.emplace(X, Y).SE;
		if(exist) { return false; }
		set<LL> dist;
		inc(i, 3) {
		inc(j, 3) {
			RF(e, v[i][j]) { dist.insert(abs(e.FI - X) + abs(e.SE - Y)); }
		}
		}
		return (SI(dist) == 1);
	};
	
	auto xy = [](LL a, LL b) -> pair<LL, LL> { return { (b + a) / 2, (b - a) / 2 }; };
	vector<LL> X = { xy(a, B).FI, xy(A, b).FI };
	vector<LL> Y = { xy(a, b).SE, xy(A, B).SE };
	sort(ALL(X));
	sort(ALL(Y));
	
	LL ans = 0;
	vector<LL> xp = { X[0] - 1, X[0], X[1], X[1] + 1 };
	vector<LL> yp = { Y[0] - 1, Y[0], Y[1], Y[1] + 1 };
	vector<LL> xl = { X[0] - 1,    1,    1, h - X[1] };
	vector<LL> yl = { Y[0] - 1,    1,    1, w - Y[1] };
	inc(i, 4) {
	inc(j, 4) {
		if(ok(xp[i], yp[j])) { ans += xl[i] * yl[j]; }
	}
	}
	if(ok(X[0] + 1, Y[0] + 1)) { ans += X[1] - X[0] - 1; }
	if(ok(X[0] + 1, Y[1] - 1)) { ans += X[1] - X[0] - 1; }
	
	cout << ans << endl;
	
	return 0;
}