#include <bits/stdc++.h>
#define MAX(a,b)   (((a)>(b))?(a):(b))
#define MIN(a,b)   (((a)>(b))?(b):(a))
#define ulli       unsigned long long int
#define lli        long long int
#define pb         push_back
#define mp         make_pair
#define fi         first
#define se         second
#define all(x)     x.begin(),x.end()
#define rall(x)    x.rbegin(),x.rend()
#define vi         vector<lli>
#define pii        pair<lli,lli>
#define itr        iterator
#define ub         upper_bound
#define lb         lower_bound
#define rep(lo,hi) for(lli i=lo;i<hi;i++)
#define traverse(container,it) for(typeof(container.begin()) it=container.begin();it!=container.end();it++)
#define mem(a,val) memset(a,val,sizeof(a))
#define gcd        __gcd
#define digits(n)  (floor(log10(n))+1)
#define bin_1(n)    __builtin_popcount(n)
#define ios        ios_base::sync_with_stdio(false);cin.tie(0)
#define out        cerr<<(float(clock())/CLOCKS_PER_SEC)<<endl
#define iss(s)     istringstream(s)
#define MOD        1000000007
#define PI         acos(-1.0)
#define eps        1e-9
#define inf        (lli)1e18+10
using namespace std;
void fastscan(int &x) { bool neg=false; register int c; x=0; c=getchar(); if(c=='-'){ neg = true; c=getchar(); } for(;(c>47 && c<58);c=getchar()) x = (x<<1) + (x<<3) +c -48; if(neg) x *=-1; }
int bin1(lli x) { int r=0; while(x) { r++; x&=x-1; } return r; }
lli lcm(lli a,lli b) { return (a/gcd(a,b))*b; }
lli exgcd(lli a,lli b,lli &x,lli &y){ if(a==0) { x=0; y=1; return b; } lli x1,y1,gcd=exgcd(b%a,a,x1,y1); x=y1-(b/a)*x1; y=x1; return gcd; }
lli modpow(lli b,lli e,lli m) { lli r=1; b=b%m; while(e>0) { if(e%2==1) r=(r*b)%m; e/=2; b=(b*b)%m; } return r; }
lli modinverse(lli a,lli mod) { return modpow(a,mod-2,mod);    }
lli modinverse2(lli a,lli m) { lli x,y; exgcd(a,m,x,y);  while(x<0) x+=m; return x; }
lli nCrmod(lli n,lli r,lli m) {if(r>n) r=n-r; lli res=1; for(lli i=r;i>=1;i--) { res=(res*(n-i+1))%m; res=(res*modinverse(i,m)); } return res; }
lli power(lli b,lli e) { if(e==0) return 1; if (e%2==0) return power(b*b,e/2); return b*power(b*b,e/2); }
double powerf(double b,int e) {if(e==0) return 1; if(e%2==0)return powerf(b*b,e/2); else{ if(e>0)return b*powerf(b*b,e/2); else return powerf(b*b,e/2)/b;}}
lli phi(lli n) { lli res=n,p; for(p=2;p*p<=n;p++){ if(n%p==0){ while(n%p==0) n/=p; res-=res/p; }} if(n>1) res-=res/n; return res; }
bool isprime(lli x) { if(x==1) return false; for(lli i=2;i*i<=x;i++) if(x%i==0) return false; return true; }
#define sievesize  1000010
bool prime[sievesize+1];
void sieve() { lli x,y; prime[0]=prime[1]=true; for(x=2;x*x<=sievesize;x++){ if(!prime[x]) for(y=x*x;y<=sievesize;y+=x) prime[y]=true; }}
bool isvowel(char c) { if(isupper(c)) c=tolower(c); return c=='a'||c=='e'||c=='i'||c=='o'||c=='u'; }
double dist(int x1,int y1,int x2,int y2) { return sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2)); }
int dx[]={0,0,-1,1};
int dy[]={-1,1,0,0};
#define N (int)1e5+10
void pre(){;}
void gcj()
{   //freopen("C:\\Users\\atul5095\\Downloads\\A-small-attempt0.in","r",stdin);
    //freopen("C:\\Users\\atul5095\\Downloads\\A-small-output.out","w",stdout);
    //freopen("C:\\Users\\atul5095\\Downloads\\A-large.in","r",stdin);
    //freopen("C:\\Users\\atul5095\\Downloads\\A-large-output.out","w",stdout);
    lli t; cin>>t;
    for(lli c=1;c<=t;c++)
    {   cout<<"Case #"<<c<<": ";
        cout<<endl;
    }
}
void judge()
{   int n,k,x,y;
    cin>>n>>k>>x>>y;
    if(n<k) cout<<n*x;
    else cout<<k*x+(n-k)*y;
}
int main()
{   ios;
    int t=1;
    //cin>>t;
    while(t--)
    {   judge();
    }
    return 0;
}


Measured memory_bytes: 256