type m=record
  q,s:longint;
end;
var
  a:array[1..5050] of m;
  t:m;
  n,l,i,j,k,ans:longint;
begin
assign(input,'milk.in');
assign(output,'milk.out');
reset(input);
rewrite(output);
  readln(l,n);
  for i:=1 to n do
    readln(a[i].q,a[i].s);
  for i:=1 to n-1 do
  begin
    for j:=i+1 to n do
      if a[i].q>a[j].q then
      begin
        t:=a[i];
        a[i]:=a[j];
        a[j]:=t;
      end;
  end;
  for i:=1 to n do
    if l<a[i].s then
      break else
    begin
      ans:=ans+a[i].s*a[i].q;
      l:=l-a[i].s;
    end;
  ans:=ans+l*a[i].q;
  writeln(ans);
  close(input);
  close(output);
end.