var t,p:array[1..2000] of longint;
n,c,i,m,b2,pmax,pmax2,x:longint;
f,g:text;
function val(pr:longint):word;
         begin
         if pr>=x then
            val:=1
         else
             val:=0;
         end;
begin
assign(f,'carnati.in');
assign(g,'carnati.out');
reset(f);rewrite(g);
readln(f,n,c);
for i:=1 to n do
    readln(f,t[i],p[i]);
pmax2:=-maxlongint;
for m:=1 to n do
    begin
    x:=p[m];
    pmax:=val(p[1])*x-c;
    b2:=pmax;
    for i:=2 to n do
        begin
        if b2+c*(t[i-1]-t[i]+1)>=0 then
           b2:=b2+val(p[i])*x+t[i-1]*c-t[i]*c
        else
            b2:=val(p[i])*x-c;
        if b2>pmax then
           pmax:=b2;
        end;
    if pmax>pmax2 then
       pmax2:=pmax;
    end;
if pmax2>0 then
   write(g,pmax2)
else
    write(g,0);
close(f);close(g);
end.