var f,g:text;
    i,n,j,t,s,suma:longint;
    m:array[0..100001] of longint;
    c,p:array[0..100001] of longint;
begin
assign(f,'branza.in');reset(f);
assign(g,'branza.out');rewrite(g);
read(f,n,s,t);
for i:=1 to n do read(f,c[i],p[i]);
for i:=1 to n do m[i]:=maxint;
for i:=1 to n do
 for j:=1 to t do
  if i-j<=t then
   if c[j]+(i-j)*s<m[i] then
    m[i]:=c[j]+(i-j)*s;
suma:=0;
for i:=1 to n do
 suma:=suma+m[i]*p[i];
writeln(g,suma);
close(f);
close(g);
end.