var
  n,s,a,sum,i,j:longint;
  c,y:array[1..10000]of longint;
  fin,fout:text;
begin
  assign(fin,'factory.in');reset(fin);
  assign(fout,'factory.out');rewrite(fout);
  read(fin,n,s);
  for i:=1 to n do
    read(fin,c[i],y[i]);
  sum:=c[1]*y[1];
  for i:=2 to n do begin
    a:=5;j:=0;
    repeat
      inc(j);
      if c[i]-a>c[i-j] then c[i]:=c[i-j]+a;
      a:=a+5;

    until i-j=1;
  end;
  for i:=2 to n do
    sum:=c[i]*y[i]+sum;
  write(fout,sum);
  close(fin);
  close(fout);
end.