PROGRAM trochoibockeo;
USES crt;
VAR f1,f2:TEXT; a:ARRAY[1..1000] OF WORD;
n,k,i,cs,s : INTEGER;
BEGIN
assign(f1,'GAME.INP');
assign(f2,'GAME.OUT');
reset(f1);
rewrite(f2);
Readln(f1,n);
Readln(f1,k);
FOR i:=1 TO n DO
Readln(f1,a[i]);    
IF k=n THEN cs:=1 ELSE
cs:=k+1;
FOR i:=1 TO n DIV 2 DO
BEGIN
s:=s+a[cs];
IF cs= n-1 THEN cs:=1 
ELSE IF cs= n THEN cs:=2 
ELSE  cs:=cs+2;
END;
Write(f2,s);
close(f2);
END.