program xmz;
var
x,y,f:array[1..10000]of longint;
n,s,a,b,c:longint;
t:array[1..50]of longint;
f1,f2:text;

procedure add(n:longint);
 var
 i:integer;
 begin
  i:=1;
  t[1]:=t[1]+n;
  repeat
     t[i+1]:=t[i+1]+t[i]div 10000;
     t[i]:=t[i]mod 10000;
     i:=i+1;
  until t[i]<10000;
  if i>c then c:=i;
 end;

begin
assign(f1,'factory.in');assign(f2,'factory.out');
reset(f1);rewrite(f2);
read(f1,n,s);
for a:=1 to n do
 read(f1,x[a],y[a]);

for a:=1 to n do f[a]:=990000000;

for a:=1 to n do
 for b:=1 to a do
  if f[a]>((a-b)*s+x[b])*y[a] then
   f[a]:=((a-b)*s+x[b])*y[a];

for a:=1 to n do
 add(f[a]);

if t[c]<>0 then write(f2,t[c]);
for a:=c-1 downto 1 do
begin
if t[a]>=1000 then write(f2,t[a]) else
if t[a]>=100  then write(f2,0,t[a]) else
if t[a]>=10   then write(f2,'00',t[a]) else
write(f2,'000',t[a]);

end;
close(f1);close(f2);
end.