#include<iostream>
using namespace std;
int f[100001];
int n, h, a, d,ans,w;
int s[100001][2];
bool df(int a, int b)
{
	double midx = ((double)a + (double)b) / 2;
	double midy = h - ((double)b - (double)a) / 2;
	if (midy < f[a]||midy<f[b])
		return 0;
	double r = ((double)b - (double)a) / 2;
	for (int i = a; i <= b; i++)
	{
		if (f[a]>=0)
		{
			if ((f[a]>=midy)&&((midx - (double)a) * (midx - (double)a) + (midy - (double)f[a]) * (midy - (double)f[a]))>(r*r))
				return 0;
		}
	}
	return 1;
}
int main()
{
	cin >> n >> h >> a >> d;
	int max1 = 0,x,fr;
	for (int i = 1; i <= 100000; i++)
		f[i] = -1;
	for (int i = 1; i <= n; i++)
	{
		cin >> x;
		cin >> f[x];
		w += a*(h - f[x]);
		ans++;
		s[ans][0] = x;
		if (f[x] > max1)
			max1 = f[x];
	}
	bool v = 0;
	for (int i = 1; i < ans; i++)
	{
		v = 0;
		for (int j = i + 1; j <= ans; j++)
		{
			if ((df(s[i][0], s[j][0])))
			{
				v = 1;
				s[i][1] = j;
				w += d * ((s[j][0] - s[i][0]) * (s[j][0] - s[i][0]));
				i = j - 1;
				break;
			}
		}
		if (!v)
		{
			cout << "impossible";
			break;
		}
	}
	if(v)
	{
		while (1)
		{
			int yy = 1;
			bool v = 1;
			while (yy != ans && s[yy][1] != ans)
			{
				if (df(s[yy][0], s[s[s[yy][1]][1]][0]))
				{
					int qw = h - f[s[s[yy][1]][0]];
					int qe = (s[s[s[yy][1]][1]][0] - s[yy][0]) * (s[s[s[yy][1]][1]][0] - s[yy][0]);
					int qe1 = (s[s[s[yy][1]][1]][0] - s[s[yy][1]][0]) * (s[s[s[yy][1]][1]][0] - s[s[yy][1]][0]);
					int qe2 = (s[s[yy][1]][0] - s[yy][0]) * ((s[s[yy][1]][0] - s[yy][0]));
					if (a * qw > d * (qe - qe1 - qe2))
					{
						v = 0;
						s[yy][1] = s[s[yy][1]][1];
						w -= a * qw - d * (qe - qe1 - qe2);
					}
				}
				yy = s[yy][1];
			}
			if (v)
				break;
		}
		cout << w;
	}
}