Pascal :

var t,tt,i,m,n,a,b:longint;
    sum:array[0..1000] of longint;
begin
  readln(n,m);
  for i:=1 to m do
    begin readln(a,b);
      inc(sum[a],b);
    end;
  for i:=0 to 1000 do
    begin if sum[i]<>0 
    then    begin 
             inc(t,i*sum[i]);
             inc(tt,sum[i]);
            end;
     if tt>=n then break;
    end;
  t:=t-(tt-n)*i;
  writeln(t);
end.