#include<iostream>
#include<algorithm>
using namespace std;
int f[100001];
long long n, h, a, d,ans;
long long s[100001][2];
long long qw[10002],qe[10002];
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;
	r *= r;
	for (int i = a; i <= b; i++)
	{
		if (f[i]>=0)
		{
			if ((f[i]>=midy)&&((midx - (double)i) * (midx - (double)i) + (midy - (double)f[i]) * (midy - (double)f[i]))>r)
				return 0;
		}
	}
	return 1;
}
int vv = 0;
long long min1 = -1,mn;
void dfs()
{
	for (int i = 2; i <= ans; i++)
	{
		for (int u = i-1; u>=1; u--)
		{
			if (qe[i] && (((s[i][0] - s[u][0]) * (s[i][0] - s[u][0])) - qe[i] > mn))
				break;
			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];
					qe[i]=((s[i][0] - s[u][0])* (s[i][0] - s[u][0]));
				}
				else
				{
					qw[i] = min(qw[i], qw[u] + d * ((s[i][0] - s[u][0]) * (s[i][0] - s[u][0])) + s[i][1]);
					qe[i] = ((s[i][0] - s[u][0]) * (s[i][0] - s[u][0]));
				}
				if (i == ans)
				{
					if (min1 > qw[i]||min1==-1)
						min1 = qw[i];
					vv = 1;
				}
			}
		}
	}
}
int main()
{
	cin >> n >> h >> a >> d;
	int x,fr;
	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]]);
	}
	qw[1] = s[1][1];
	mn = h * a;
	dfs();
	if (!vv)
		cout << "impossible";
	if(vv)
		cout << min1 << endl;
}