gistfile1.java

import java.io.*;
import java.util.*;

import static java.lang.Math.*;

public class Solution {

	BufferedReader in;
	StringTokenizer st;
	PrintWriter out;
	String filename = "rag";

	static class Point {
		double x;
		int y;

		public Point(double x, int y) {
			super();
			this.x = x;
			this.y = y;
		}
	}

	int X, Y;
	int n;
	Point[] v;

	void solve() throws IOException {
		X = ni();
		Y = ni();
		n = ni();
		v = new Point[n];
		for (int i = 0; i < n; ++i) {
			int x = ni();
			int y = ni();
			v[i] = new Point(x, y);
		}
		Point[] stack = new Point[n + 1];
		int sz = 0;
		stack[sz++] = v[0];
		stack[sz++] = v[1];
		for (int i = 2; i < n; ++i) {
			if (stack[sz - 1].y < v[i].y && v[i].x > stack[sz - 1].x) {
				while (sz > 0 && stack[sz - 1].y < v[i].y
						&& v[i].x > stack[sz - 1].x)
					--sz;
				double ty = stack[sz].y - stack[sz - 1].y;
				double tx = stack[sz].x - stack[sz - 1].x;
				double dx = (v[i].y - stack[sz - 1].y) * tx / ty;
				stack[sz] = new Point(stack[sz - 1].x + dx, v[i].y);
				sz++;
			}
			stack[sz++] = v[i];
		}

		// reverse
		v = stack.clone();
		sz = 0;
		stack[sz++] = v[n - 1];
		stack[sz++] = v[n - 2];
		for (int i = n - 3; i >= 0; --i) {
			if (stack[sz - 1].y > v[i].y && v[i].x > stack[sz - 1].x) {
				while (sz > 0 && stack[sz - 1].y > v[i].y
						&& v[i].x > stack[sz - 1].x)
					--sz;
				double ty = stack[sz].y - stack[sz - 1].y;
				double tx = stack[sz].x - stack[sz - 1].x;
				double dx = (v[i].y - stack[sz - 1].y) * tx / ty;
				stack[sz] = new Point(stack[sz - 1].x + dx, v[i].y);
				sz++;
			}
			stack[sz++] = v[i];
		}

		stack[sz++] = new Point(X, Y);

		double ret = 0.0;
		for (int i = 0; i < sz; ++i) {
			int j = (i + 1) % sz;
			Point p1 = stack[i], p2 = stack[j];
			ret += (p1.y + p2.y) * (p2.x - p1.x);
		}
		ret *= 0.5;
		out.println(abs(ret));
	}

	public Solution() throws IOException {
		Locale.setDefault(Locale.US);
		in = new BufferedReader(new FileReader(filename + ".in"));
		out = new PrintWriter(filename + ".out");
		solve();
		in.close();
		out.close();
	}

	String ns() throws IOException {
		while (st == null || !st.hasMoreTokens())
			st = new StringTokenizer(in.readLine());
		return st.nextToken();
	}

	int ni() throws IOException {
		return Integer.valueOf(ns());
	}

	long nl() throws IOException {
		return Long.valueOf(ns());
	}

	double nd() throws IOException {
		return Double.valueOf(ns());
	}

	public static void main(String[] args) throws IOException {
		new Solution();
	}
}