#include<bits/stdc++.h>
#define f first
#define s second
#define ll long long
#define pb push_back
using namespace std;

const ll oo=LLONG_MAX/2;
vector<pair<int,int>> sols(int x,int y){
    ll k=2*(ll)x*y;
    long double a=sqrtl(k);
    ll sq=-1;
    for(ll x=a-2;x<=a+2;x++){
        if(x*x==k){
            sq=x;
        }
    }
    int badL=-1,badR=-1;
    if(sq!=-1){
        badL=2*(x+y-sq)-1;
        badR=2*(x+y+sq)+1;
    }
    else{
        badL=floor(2*(x+y-a));
        badR=ceil(2*(x+y+a));
    }
    badL=max(badL,0);
    y=y*2;
    y=max(y,x-1);
    if(badL>y){
        return {{y+1,badL},{badR,INT_MAX}};
    }
    else{
        if(badR>y){
            return {{badR,INT_MAX}};
        }
        else{
            return {{y+1,INT_MAX}};
        }
    }
}
int main(){
    int n,h,a,b;
    cin>>n>>h>>a>>b;
    vector<ll> dp(n,oo);
    vector<int> x(n),y(n);
    for(int i=0;i<n;i++){
        cin>>x[i]>>y[i];
        y[i]=h-y[i];
    }
    const int N=1e5+5;
    vector<int> next(N,INT_MAX);
    for(int i=0;i<n;i++){
        next[x[i]]=i;
    }
    for(int i=N-2;i>=0;i--){
        next[i]=min(next[i],next[i+1]);
    }
    dp[n-1]=(ll)a*y[n-1];
    vector<int> c(n);
    for(int i=n-2;i>=0;i--){
        for(int j=i+1;j<n;j++)x[j]-=x[i],c[j]=0;
        int cntBad=0;
        for(int j=i+1;j<n;j++){
            if(x[j]>2*y[i])break;
            /*cout << i << " " << j << endl;
            for(auto p:sols(x[j],y[j])){
                cout << "bad " << p.f << " " << p.s << endl;
            }*/
            for(auto p:sols(x[j],y[j])){
                p.f+=x[i];
                if(p.s!=INT_MAX)
                    p.s+=x[i]+1;
                if(p.f<N&&next[p.f]!=INT_MAX){
                    c[next[p.f]]++;
                }
                if(p.s<N&&p.s!=INT_MAX&&next[p.s]!=INT_MAX){
                    c[next[p.f]]--;
                }
            }
            cntBad+=c[j];
            if(cntBad==0){
                dp[i]=min(dp[i],dp[j]+(ll)x[j]*x[j]*b+(ll)y[i]*a);
            }
        }



        for(int j=i+1;j<n;j++)x[j]+=x[i];
    }
    if(dp[0]>=oo){
        printf("impossible\n");
    }
    else
        printf("%lld\n",dp[0]);
}