Codeforces 466 A. Cheap Travel

A. Cheap Travel

time limit per test
1 second

memory limit per test
256 megabytes

input
standard input

output
standard output

Ann has recently started commuting by subway. We know that a one ride subway ticket costs a rubles. Besides, Ann found out that she can buy a special ticket
for m rides (she can buy it several times). It costs b rubles.
Ann did the math; she will need to use subway n times. Help Ann, tell her what is the minimum sum of money she will have to spend to make n rides?

Input

The single line contains four space-separated integers n, m, a, b (1 ≤ n, m, a, b ≤ 1000)
— the number of rides Ann has planned, the number of rides covered by the m ride ticket, the price of a one ride ticket and the price of an m ride
ticket.

Output

Print a single integer — the minimum sum in rubles that Ann will need to spend.

Sample test(s)

input

6 2 1 2

output

6

input

5 2 2 3

output

8

Note

In the first sample one of the optimal solutions is: each time buy a one ride ticket. There are other optimal solutions. For example, buy threem ride tickets.

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>

using namespace std;

int main()
{
long long int n,m,a,b;
cin>>n>>m>>a>>b;
long long int k=n/m;
long long int ans=n*a;
for(int i=0;i*m<2*n;i++)
{
int r=n-m*i;
if(r<0) r=0;
ans=min(ans,r*a+i*b);
}
if(n%m==0)
ans=min(ans,k*b);
else
ans=min(ans,(k+1)*b);
cout<<ans<<endl;
return 0;
}