// Milk and Honey
package Bronze_III_3;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;
public class Ex7130 {
public static void main(String[] args) throws IOException {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
StringTokenizer st = new StringTokenizer(br.readLine());
int m = Integer.parseInt(st.nextToken()); // milk
int h = Integer.parseInt(st.nextToken()); // honey
int n = Integer.parseInt(br.readLine()); // number of fields
int total = 0;
for (int i = 0; i < n; i++) {
st = new StringTokenizer(br.readLine());
int c = Integer.parseInt(st.nextToken()); // cow
int b = Integer.parseInt(st.nextToken()); // bee
total += Math.max(m * c, h * b);
}
System.out.println(total);
}
}