import java.util.*;
class PairSome {
public long tk;
public int ck;
PairSome(long tk, int ck) {
this.ck = ck;
this.tk = tk;
}
}
public class ThreeDimMatrix {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
long rows = in.nextLong();
long cols = in.nextLong();
int height = in.nextInt();
long cells = rows * cols;
long sum = 0;
List<PairSome> list = new ArrayList<PairSome>();
long tk;
int ck;
for (int i = 0; i < height; i++) {
tk = in.nextLong();
ck = in.nextInt();
sum += tk;
list.add(new PairSome(tk, ck));
}
if (sum < cells) {
System.out.println("Impossible");
return;
}
long minCost = 0;
Comparator comparator = new Comp();
if (list != null && !list.isEmpty()) {
Collections.sort(list, comparator);
long cellsPainted = 0;
for (PairSome pairSome : list) {
if (cellsPainted + pairSome.tk < cells) {
minCost += (pairSome.tk * pairSome.ck);
cellsPainted += pairSome.tk;
} else {
minCost += ((cells - cellsPainted) * pairSome.ck);
break;
}
}
}
System.out.println(minCost);
}
}
class Comp implements Comparator {
@Override
public int compare(PairSome e1, PairSome e2) {
if (e1.ck > e2.ck) {
return 1;
} else {
return -1;
}
}
}
}