#include <bits/stdc++.h>

using namespace std;

typedef long double ld;
typedef long long ll;

const int maxn = 10240;

ll x[maxn], y[maxn];
ll dp[maxn];

int main() {
    int n;
    ll h, alpha, beta;
    cin >> n >> h >> alpha >> beta;
    for (int i = 1; i <= n; i++) {
        cin >> x[i] >> y[i];
    }
    dp[1] = alpha * (h - y[1]);
    for (int i = 2; i <= n; i++) {
        dp[i] = 1ll << 61;
    }
    for (int i = 2; i <= n; i++) {
        ld lower = 0;
        ld upper = h - y[i];
        for (int j = i-1; j >=1;  j--) {
            ld s = h - y[j];
            ld t = x[i] - x[j];
            ld delta = sqrt(2 * s * t);
            ld ll = s + t - delta;
            ld rr = s + t + delta;
            if (t <= 2 * s) {
                upper = min(upper, rr);
            } else {
                lower = max(lower, ll);
                upper = min(upper, rr);
            }
            ld r = (x[i] - x[j]) / 2.0;
            if (lower - 1e-9 <= r && r <= upper + 1e+9 && r <= h - y[j]) {
                dp[i] = min(dp[i], dp[j] + alpha * (h - y[i]) + beta * (x[i] - x[j]) * (x[i] - x[j]));
            }
        }
    }
    if (dp[n] >= 1ll << 60) {
        cout << "impossible\n";
    } else {
        cout << dp[n] << endl;
    }
}