import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }




void main() {
  try {
    for (; ; ) {
      const W = readLong();
      const H = readLong();
      const N = readInt();
      auto X = new long[N];
      auto Y = new long[N];
      foreach (i; 0 .. N) {
        X[i] = readLong();
        Y[i] = readLong();
      }
      
      auto A = new long[N];
      auto B = new long[N];
      foreach (i; 0 .. N) {
        A[i] = X[i] + Y[i];
        B[i] = -X[i] + Y[i];
      }
      const as = A.dup.sort.uniq.array;
      const bs = B.dup.sort.uniq.array;
      debug {
        writeln("A = ", A);
        writeln("B = ", B);
        writeln("as = ", as);
        writeln("bs = ", bs);
      }
      
      bool check(long ca, long cb, long sz) {
        const cx = (ca - cb) / 2;
        const cy = (ca + cb) / 2;
        if (!(1 <= cx && cx <= W && 1 <= cy && cy <= H)) {
          return false;
        }
        foreach (i; 0 .. N) {
          if (sz != max(abs(A[i] - ca), abs(B[i] - cb))) {
            return false;
          }
        }
        debug {
          writeln("check ", ca, " ", cb, " ", sz, " = true");
        }
        return true;
      }
      
      long ans;
      if (A.map!(a => (a & 1)).array.sort.uniq.array.length == 1 &&
          B.map!(a => (a & 1)).array.sort.uniq.array.length == 1) {
        if (as.length == 1 && bs.length == 1) {
          ans = H * W;
        } if (as.length == 1) {
          /*
                 ##
                 ##
              o
               o
                o
            ##
            ##
          */
          const i0 = X.minIndex;
          const i1 = X.maxIndex;
          ans += (X[i0] - 1 + 1) * (Y[i1] - 1 + 1);
          ans += (W - X[i1] + 1) * (H - Y[i0] + 1);
          if (N == 2) {
            ans += (X[i1] - X[i0] - 1);
          }
        } else if (bs.length == 1) {
          /*
            ##
            ##
                o
               o
              o
                 ##
                 ##
          */
          const i0 = X.minIndex;
          const i1 = X.maxIndex;
          ans += (X[i0] - 1 + 1) * (H - Y[i1] + 1);
          ans += (W - X[i1] + 1) * (Y[i0] - 1 + 1);
          if (N == 2) {
            ans += (X[i1] - X[i0] - 1);
          }
        } else {
          // no as[0], as[$ - 1]
          if (bs.length == 2) {
            const sz = (bs[$ - 1] - bs[0]) / 2;
            const cb = bs[0] + sz;
            // x = 1 or y = 1
            long aL = max(1 + (cb + 1), (1 - cb) + 1);
            // x = W or y = H
            long aU = min(W + (cb + W), (H - cb) + H);
            foreach (i; 0 .. N) {
              chmax(aL, A[i] - sz);
              chmin(aU, A[i] + sz);
            }
            chmax(aL, as[$ - 1] - sz + 2);
            chmin(aU, as[0] + sz - 2);
            ans += max((aU - aL) / 2 + 1, 0);
          }
          // no bs[0], bs[$ - 1]
          if (as.length == 2) {
            const sz = (as[$ - 1] - as[0]) / 2;
            const ca = as[0] + sz;
            // x = W or y = 1
            long bL = max(-W + (ca - W), -(ca - 1) + 1);
            // x = 1 or y = H
            long bU = min(-1 + (ca - 1), -(ca - H) + H);
            foreach (i; 0 .. N) {
              chmax(bL, B[i] - sz);
              chmin(bU, B[i] + sz);
            }
            chmax(bL, bs[$ - 1] - sz + 2);
            chmin(bU, bs[0] + sz - 2);
            ans += max((bU - bL) / 2 + 1, 0);
          }
          // [as[0], *] x [bs[0], *]
          {
            long sz;
            long yL = (as[0] + bs[0]) / 2;
            foreach (i; 0 .. N) {
              if (A[i] == as[0] || B[i] == bs[0]) {
                chmax(yL, Y[i]);
              } else {
                chmax(sz, max(A[i] - as[0], B[i] - bs[0]) / 2);
              }
            }
            if (sz) {
              if (check(as[0] + sz, bs[0] + sz, sz)) ans += 1;
            } else {
              ans += max(H - yL + 1, 0);
            }
          }
          // [as[0], *] x [*, bs[$ - 1]]
          {
            long sz;
            long xL = (as[0] - bs[$ - 1]) / 2;
            foreach (i; 0 .. N) {
              if (A[i] == as[0] || B[i] == bs[$ - 1]) {
                chmax(xL, X[i]);
              } else {
                chmax(sz, max(A[i] - as[0], bs[$ - 1] - B[i]) / 2);
              }
            }
            if (sz) {
              if (check(as[0] + sz, bs[$ - 1] - sz, sz)) ans += 1;
            } else {
              ans += max(W - xL + 1, 0);
            }
          }
          // [as[0], *] x [bs[0], bs[$ - 1]]
          {
            const sz = (bs[$ - 1] - bs[0]) / 2;
            if (check(as[0] + sz, bs[0] + sz, sz)) ans -= 1;
          }
          // [*, as[$ - 1]] x [bs[0], *]
          {
            long sz;
            long xU = (as[$ - 1] - bs[0]) / 2;
            foreach (i; 0 .. N) {
              if (A[i] == as[$ - 1] || B[i] == bs[0]) {
                chmin(xU, X[i]);
              } else {
                chmax(sz, max(as[$ - 1] - A[i], B[i] - bs[0]) / 2);
              }
            }
            if (sz) {
              if (check(as[$ - 1] - sz, bs[0] + sz, sz)) ans += 1;
            } else {
              ans += max(xU - 1 + 1, 0);
            }
          }
          // [*, as[$ - 1]] x [*, bs[$ - 1]]
          {
            long sz;
            long yU = (as[$ - 1] + bs[$ - 1]) / 2;
            foreach (i; 0 .. N) {
              if (A[i] == as[$ - 1] || B[i] == bs[$ - 1]) {
                chmin(yU, Y[i]);
              } else {
                chmax(sz, max(as[$ - 1] - A[i], bs[$ - 1] - B[i]) / 2);
              }
            }
            if (sz) {
              if (check(as[$ - 1] - sz, bs[$ - 1] - sz, sz)) ans += 1;
            } else {
              ans += max(yU - 1 + 1, 0);
            }
          }
          // [*, as[$ - 1]] x [bs[0], bs[$ - 1]]
          {
            const sz = (bs[$ - 1] - bs[0]) / 2;
            if (check(as[$ - 1] - sz, bs[0] + sz, sz)) ans -= 1;
          }
          // [as[0], as[$ - 1]] x [bs[0], *]
          {
            const sz = (as[$ - 1] - as[0]) / 2;
            if (check(as[0] + sz, bs[0] + sz, sz)) ans -= 1;
          }
          // [as[0], as[$ - 1]] x [*, bs[$ - 1]]
          {
            const sz = (as[$ - 1] - as[0]) / 2;
            if (check(as[0] + sz, bs[$ - 1] - sz, sz)) ans -= 1;
          }
          // [as[0], as[$ - 1]] x [bs[0], bs[$ - 1]]
          if (as[$ - 1] - as[0] == bs[$ - 1] - bs[0]) {
            const sz = (as[$ - 1] - as[0]) / 2;
            if (check(as[0] + sz, bs[0] + sz, sz)) ans += 1;
          }
        }
      }
      writeln(ans);
    }
  } catch (EOFException e) {
  }
}