#include<bits/stdc++.h>
#define LD  double
#define cp const point
#define cl const line
const int maxn=1e4+10;
const LD INF=1e18;
const LD eps=1e-8;
int sgn(LD x)
{
    return x > eps ? 1 : (x < -eps ? -1 : 0);
}
LD sqr(LD x)
{ return x * x; }
int X[maxn],Y[maxn];
LD L[maxn],R[maxn];
long long H,A,B;
int main()
{
    std::ios::sync_with_stdio(false);
    std::cin.tie(0),std::cout.tie(0);
    int n;
    std::cin>>n>>H>>A>>B;
    for(int i=1;i<=n;i++)
        std::cin>>X[i]>>Y[i];
    std::vector<long long>dp(n+1,4e18);
    for(int i=1;i<=n;i++)
    {
        if(i!=n)
        {
            R[i]=std::min((LD)(X[n]-X[i])/2,(LD)H-Y[i]);
            for(int j=i-1;j>=1;j--)
            {
                if(X[j]-X[i]>R[i])break;
                LD t0=Y[j]-H;
                LD t2=X[i]-X[j];
                LD b=2*(t0+t2),c=t2*t2+t0*t0;
                LD r=(-b+sqrtl(b*b-4*c))/2;
                R[i]=std::min(R[i],std::min(-t0,r));
            }
        }
        if(i!=1)
        {
            L[i]=std::min((LD)(X[i]-X[1])/2,(LD)H-Y[i]);
            for(int j=i-1;j>=1;j--)
            {
                if(X[i]-X[j]>L[i])break;
                LD t0=Y[j]-H;
                LD t2=X[j]-X[i];
                LD b=2*(t0+t2),c=t2*t2+t0*t0;
                LD r=(-b+sqrtl(b*b-4*c))/2;
                L[i]=std::min(L[i],std::min(-t0,r));
            }
        }
//        std::cout<<L[i]<<' '<<R[i]<<std::endl;
    }
    dp[1]=A*(H-Y[1]);
    for(int i=2;i<=n;i++)
    {
        for(int j=i-1;j>=1;j--)
        {
            if(R[j]>=(LD)(X[i]-X[j])/2&&L[i]>=(LD)(X[i]-X[j])/2)
                dp[i]=std::min(dp[i],dp[j]+A*(H-Y[i])+B*(X[i]-X[j])*(X[i]-X[j]));
        }
    }
    if(dp[n]>=dp[0])
        std::cout<<"impossible"<<std::endl;
    else std::cout<<dp[n]<<std::endl;

}