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

public class acf_yuizumi {
	private static final double EPS = 1e-12;

	public static void main(String[] args) {
		Scanner cin = new Scanner(System.in);
		while(doCase(cin)) ;
	}

	private static boolean doCase(Scanner cin) {
		int n = cin.nextInt();
		double r = cin.nextDouble();

		if(n == 0 && r == 0) return false;

		double[] vx = new double[n];
		double[] vy = new double[n];

		for(int i = 0; i < n; i++) {
			vx[i] = (double)(cin.nextInt());
			vy[i] = (double)(cin.nextInt());
		}

		double[] x = new double[2*n];
		System.arraycopy(vx, 0, x, 0, n);
		for(int i = 0; i < n; i++) { x[i+n] = vx[i] - r; }
		Arrays.sort(x);

		Function f = new Function(vx, vy);
		double S = 0.0;

		for(int i = 1; i < 2 * n; i++) {
			double x1 = x[i-1];
			double x2 = 0.5 * (x[i-1] + x[i]);
			double x3 = x[i];

			double y1 = f.value(x1) * f.value(x1 + r);
			double y2 = f.value(x2) * f.value(x2 + r);
			double y3 = f.value(x3) * f.value(x3 + r);

			// Simpson's rule gives the exact value since the integrand is
			// quadrandic
			S += (y1 + 4.0 * y2 + y3) * (x3 - x1);
		}

		System.out.println(S / 6.0);
		return true;
	}

	private static class Function {
		private double[] vx;
		private double[] vy;

		public Function(double[] vx, double[] vy) {
			if(vx.length != vy.length) {
				throw new IllegalArgumentException("length not match");
			}

			this.vx = vx;
			this.vy = vy;
		}

		public double value(double x) {
			if(x > vx[0]) {
				for(int i = 1; i < vx.length; i++) {
					if(x <= vx[i]) {
						double a = (vy[i] - vy[i-1]) / (vx[i] - vx[i-1]);
						return a * (x - vx[i]) + vy[i];
					}
				}
			}

			return 0.0;
		}
	}
}