void main(){
  import std.stdio, std.string, std.conv, std.algorithm;
  import std.array, std.range;

  int n, m; rd(n, m);
  auto a=new int[](n);
  foreach(i; 0..n) rd(a[i]);

  auto x=a[0];
  a.popFront;
  sort(a);

  int count(int[] b, int s){
    int cnt=0;
    while(b.length>0){
      auto y=b.back; b.popBack;
      auto p=assumeSorted(b);
      auto ub=p.upperBound(s-y);
      if(ub.length==0) break;
      b=b[($-ub.length+1)..$];
      cnt++; 
    }
    return cnt;
  }

  auto c1=count(a[0..($-1)], x+a.back);
  auto c2=count(a[1..$], x+a.front);
  if(c1>=m){writeln(-1); return;}
  if(c2<m){writeln(a.front); return;}

  int ng=0, ok=n-1;
  while(ok-ng>1){
    auto md=(ok+ng)/2;
    auto cnt=count(a[0..md]~a[(md+1)..$], x+a[md]);
    cnt<m ? ok : ng = md;
  }

  writeln(a[ok]);
}

void rd(T...)(ref T x){
  import std.stdio, std.string, std.conv;
  auto l=readln.split;
  assert(l.length==x.length);
  foreach(i, ref e; x){
    e=l[i].to!(typeof(e));
  }
}