#include<iostream>
#include<algorithm>
using namespace std;
int f[100001];
long long n, h, a, d,ans, min1 = -1;
long long s[100001][2], qw[10002];
bool vv = 0;
struct po
{
	int a, b, max1 = -1;
}s1[200002];
int bul(int le, int ri, int i)
{
	if (le == ri)
	{
		s1[i].max1 = f[le];
		return f[le];
	}
	int a1 = bul(le, (le + ri) / 2, 2 * i);
	int a2 = bul((le + ri) / 2 + 1, ri, 2 * i + 1);
	if (a1 < a2)
		a1 = a2;
	s1[i].max1 = a1;
	s1[i].a = le;
	s1[i].b = ri;
	return a1;
}
bool fin(int le, int ri, long long y, long long x, long long r,int i)
{
	if (2*s1[i].max1 < y)
		return 1;
	if (le == ri)
	{
		if (((x - (le*2)) * (x - (le*2)) + (y - (f[le]*2)) * (y - (f[le]* 2))) > r)
			return 0;
		return 1;
	}
	return (fin(le, (le + ri) / 2, y, x, r, 2 * i) && fin((le + ri) / 2 + 1, ri, y, x, r, 2 * i + 1));
}
bool df(int a, int b)
{
	long long midy = (h*2) -(b - a);
	if (midy < (f[a]*2) || midy < (f[b]*2))
		return 0;
	long long midx = (a + b);
	long long r = (b -a);
	r *= r;
	return fin(a, b, midy, midx, r, 1);
}
void dfs()
{
	for (int i = 2; i <= ans; i++)
	{
		for (int u = i-1; u>=1; u--)
		{
			if (qw[u]&&(df(s[u][0], s[i][0])))
			{
				if (!qw[i])
					qw[i] = qw[u] + d * ((s[i][0] - s[u][0]) * (s[i][0] - s[u][0])) + s[i][1];
				else
					qw[i] = min(qw[i], qw[u] + d * ((s[i][0] - s[u][0]) * (s[i][0] - s[u][0])) + s[i][1]);
				if (i == ans)
				{
					if (min1 > qw[i]||min1==-1)
						min1 = qw[i];
					vv = 1;
				}
			}
		}
	}
}
int main()
{
	cin >> n >> h >> a >> d;
	int x,fr=0;
	ans = n;
	for (int i = 1; i <= 100000; i++)
		f[i] = -1;
	for (int i = 1; i <= n; i++)
	{
		scanf("%d", &x);
		scanf("%d", &f[x]);
		s[i][0] = x;
		s[i][1] = a * (h - f[s[i][0]]);
		if (x > fr)
			fr = x;
	}
	bul(1, fr, 1);
	qw[1] = s[1][1];
	dfs();
	if (!vv)
		cout << "impossible";
	if(vv)
		cout << min1 << endl;
}