{
ID:angelxi1
PROG:milk
LANG:PASCAL
}
var
 total,i,j,k,m,n:longint;
 money:array[1..5000,1..2]of longint;
procedure init;
begin
 readln(n,m);
 for i:=1 to m do
 for j:=1 to 2 do
 read(money[i,j]);
end;

procedure sort(l,r:longint);
var
 i,j,x,y:longint;
begin
 i:=l;j:=r;
 x:=money[((l+r)shr 1),1];
 repeat
 while money[i,1]<x do
 inc(i);
 while money[j,1]>x do
 dec(j);
 if i<=j then
 begin
 y:=money[i,1];money[i,1]:=money[j,1];money[j,1]:=y;
 y:=money[i,2];money[i,2]:=money[j,2];money[j,2]:=y;
 inc(i);dec(j);
 end;
 until i>j;
 if l<j then sort(l,j);
 if i<r then sort(i,r);
end;

begin
assign(input,'milk.in');
assign(output,'milk.out');
reset(input);
rewrite(output);
init;
total:=0;
sort(1,m);
for i:=1 to m do
 begin
  k:=money[i,1]*money[i,2];
  if (n-money[i,2])>0 then
              begin
               total:=total+k;
               n:=n-money[i,2];
              end
             else
              begin
               total:=total+n*money[i,1];
               writeln(total);
               halt;
              end;
 end;
 if total=0 then writeln(0);
 close(input);
 close(output);
end.