type    nodd=record
                x,d:longint;
        end;

var     c:array[0..5010]of longint;
        a:array[0..5010]of nodd;
        n,m,t,k,i,nn,sum:longint;

procedure down(t:longint);
var     x,d,tt:longint;
begin
        x:=a[t].x; d:=a[t].d;
        while t+t<=nn do
        begin
                tt:=t+t; if (tt<nn)and(a[tt].d<a[tt+1].d) then inc(tt);
                if a[tt].d<=d then break;
                a[t]:=a[tt]; t:=tt;
        end;
        a[t].x:=x; a[t].d:=d;
end;

procedure up(t:longint);
var     x,d:longint;
begin
        x:=a[t].x; d:=a[t].d;
        while t>1 do
        begin
                if a[t shr 1].d>=d then break;
                a[t]:=a[t shr 1]; t:=t shr 1;
        end;
        a[t].x:=x; a[t].d:=d;
end;

procedure init;
begin
        readln(n,m,t,k);
        m:=m-(n-1)*t-1; nn:=0;
        for i:=1 to n do
        begin
                read(t,c[i]);
                if t>0 then
                begin
                        inc(nn); a[nn].x:=i; a[nn].d:=t;
                        up(nn);
                end;
        end;
end;

procedure work;
begin
        sum:=0;
        while m>=k do
        begin
                m:=m-k; sum:=sum+a[1].d;
                if a[1].d>c[a[1].x] then dec(a[1].d,c[a[1].x]) else
                begin
                        a[1]:=a[nn]; dec(nn); if nn<1 then break;
                end;
                down(1);
        end;
        writeln(sum);
end;

begin
assign(input,'fisha.in');reset(input);
assign(output,'fisha.out');rewrite(output);
        init;
        work;
close(output);
end.