var f:text; n,s,min,x,xx,l,t:int64;
p,q,sol:array[1..100000] of int64;
i,j,k:longint;
procedure citire;
begin
assign(f,'branza.in');reset(f);
read(f,n,s,t);
for i:=1 to n do
          read(f,p[i],q[i]);
close(f);
end;
begin
citire;
for i:=n downto 1 do
        begin
        if i-t>0 then min:=p[i-t]+t*s
                else min:=p[1]+i*s;
        k:=i-t+1;
        if i-t+1<=0 then k:=1;
        for j:=k to i do
                begin
                x:=p[j]+(i-j)*s;
                if x<min then min:=x;
                end;
        sol[i]:=min;
        end;
assign(f,'branza.out');rewrite(f);
s:=0;
for i:=1 to n do
        s:=s+q[i]*sol[i];
writeln(f,s);
close(f);
end.