Problem:
Score : 100 points

Problem Statement

There is a hotel with the following accommodation fee:

X yen (the currency of Japan) per night, for the first K nights

Y yen per night, for the (K+1)-th and subsequent nights

Tak is staying at this hotel for N consecutive nights.
Find his total accommodation fee.

Constraints

1 \leq N, K \leq 10000

1 \leq Y < X \leq 10000

N,\,K,\,X,\,Y are integers.

Input

The input is given from Standard Input in the following format:

N
K
X
Y

Output

Print Tak's total accommodation fee.

Sample Input 1

5
3
10000
9000

Sample Output 1

48000

The accommodation fee is as follows:

10000 yen for the 1-st night

10000 yen for the 2-nd night

10000 yen for the 3-rd night

9000 yen for the 4-th night

9000 yen for the 5-th night

Thus, the total is 48000 yen.

Sample Input 2

2
3
10000
9000

Sample Output 2

20000

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>


using namespace std;


#define ll long long
#define mk make_pair
#define pb push_back
#define fi first
#define se second
#define all(x) x.begin(), x.end()
#define sz(x) (int) (x).size()


ll sum(int n);


void go(){
	int n,k,x,y;cin>>n>>k>>x>>y;
	int ans=0;
	for(int i=1;i<=n;i++){
		if(i<=k){
			ans+=x;
		}else{
			ans+=y;
		}
	}
	cout << ans;
}
int main(){
   
   ios_base::sync_with_stdio(0);
   cin.tie(0);
   cout.tie(0);

   go();

   return 0;
}

ll sum(int n){                
   if(n == 0){
      return n;
   }else{
      return sum(n/10) + n%10;
   }
}