var f,g:text;
    s,m,n,k,d,x,y:longint;

begin
assign(f,'transport.in');
reset(f);
readln(f,n,k);
read(f,m);
s:=m;
y:=m;
for n:=2 to n do begin
   readln(f,x);
   s:=s+x;
   if x>m then m:=x;
   d:=d+abs(x-y);
   y:=x;
end;
s:=s div k;
d:=d div (n-1);
s:=s+d;
if s<m then s:=m;
assign(g,'transport.out');
rewrite(g);
write(g,s);
close(g);
end.