#include<bits/stdc++.h>
#include<algorithm>
#define int long long
using namespace std;
signed main(){
	//freopen(".in","r",stdin);
	//freopen(".out","w",stdout);
	//MessageBox(GetForegroundWindow(),"标题","内容",MB_YESNO);
	int a,b,x,y;
	cin>>a>>b>>x>>y;
	int sum=0;
	if(x+x<y){
		y=x+x;
	}
	if(a>b){
		sum=(a-b-1)*y;
		sum+=x;
	}else if(a==b){
		sum=x;
	}else{
		sum=(b-a)*y;
		sum+=x;
	}
	cout<<sum<<endl;
	return 0;
}

#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define rng(a) a.begin(),a.end()
#define rrng(a) a.rbegin(),a.rend()
#define isin(x,l,r) ((l) <= (x) && (x) < (r))
#define pb push_back
#define eb emplace_back
#define sz(x) (int)(x).size()
#define pcnt __builtin_popcountll
#define uni(x) x.erase(unique(rng(x)),x.end())
#define snuke srand((unsigned)clock()+(unsigned)time(NULL));
#define show(x) cerr<<#x<<" = "<<x<<endl;
#define PQ(T) priority_queue<T,v(T),greater<T> >
#define bn(x) ((1<<x)-1)
#define dup(x,y) (((x)+(y)-1)/(y))
#define newline puts("")
using namespace std;
typedef long long int ll;
typedef unsigned uint;
typedef unsigned long long ull;
typedef pair<int,int> P;
typedef tuple<int,int,int> T;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<P> vp;
typedef vector<T> vt;
const double eps = 1e-10;
const ll LINF = 1001002003004005006ll;
const int INF = 1001001001;
#define dame { puts("-1"); return 0;}
#define yn {puts("Yes");}else{puts("No");}
const int MX = 200005;


int main() {
  int a,b,x,y; cin >> a >> b >> x >> y;
  int ans;
  if(a>b){
    ans = min(((a-b)*2-1)*x, x+(a-b-1)*y);
  }else if(a == b){ 
    ans = x;
  }else{
    ans = min(((b-a)*2+1)*x, x+(b-a)*y);
  }
  cout << ans << endl;
  return 0;
}
