#include <bits/stdc++.h>
using namespace std;

#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define RFOR(i, b, a) for (int i = (b) - 1; i >= (a); i--)
#define SZ(a) int(a.size())
#define ALL(a) a.begin(), a.end()
#define PB push_back
#define MP make_pair
#define F first
#define S second

typedef unsigned long long LL;
typedef vector<int> VI;
typedef pair<int, int> PII;
typedef double db;

#define beta fkldjgkldfjg

const int N = 10'047;
const db EPS = 1e-9;
const LL LINF = 1.1e18;

int n, h;
LL alpha, beta;
int x[N], y[N];
db mxR[2][N];
LL dp[N];

int main()
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout << fixed << setprecision(15);
	cin >> n >> h >> alpha >> beta;
	FOR(i, 0, n)
		cin >> x[i] >> y[i];
	FOR(it, 0, 2)
	{
		FOR(i, 0, n)
		{
			mxR[it][i] = min((db)h - y[i], (x[n - 1] - x[i]) / 2.0);
			FOR(j, i + 1, n)
			{
				#warning
				// EPS
				if (x[j] - x[i] > 2 * mxR[it][i] + EPS)
					break;
				
				if (y[j] <= y[i])
					continue;
					
				db b = 2 * (x[i] - x[j] - h + y[j]), c = (db)(x[i] - x[j]) * (x[i] - x[j]) + (db)(h - y[j]) * (h - y[j]);
				db D = b * b - 4 * c;
				if (D < EPS) 
					continue;
				db res = (-b + sqrt(D)) / 2;
				mxR[it][i] = min(mxR[it][i], res);
			}
		}
		reverse(x, x + n);
		FOR(i, 0, n)
			x[i] *= -1;
	}
	reverse(mxR[1], mxR[1] + n);
	dp[0] = alpha * (h - y[0]);
	FOR(i, 1, n)
	{
		dp[i] = LINF;
		FOR(j, 0, i)
		{
			if (x[i] - x[j] < 2 * min(mxR[0][j], mxR[1][i]) + EPS)
				dp[i] = min(dp[i], dp[j] + alpha * (h - y[i]) + beta * (x[i] - x[j]) * (x[i] - x[j]));
		}
	}
	if (dp[n - 1] == LINF)
		cout << "impossible\n";
	else
		cout << dp[n - 1] << "\n";
	return 0;
}