#include<stdio.h>

int main(void)
{
  int N, M;
  int a[100000]; //得点
  int b[1000][1000];
  int c[100000];
  int p,q,i,j,w,h,r;
  
  
  scanf("%d %d", &N, &M);

  int acount = 0;

  
  for(i = 0; i < N ; i++){
    scanf("%d", &a[i]);
  }

  i = 0;
  j = 0;
  p = 1;
  w = 0;
 
  for(i = 0, p = 1; i < N-1; i++){
    for(j = 1; j <= N-p; j++){
      b[i][j] = a[i] + a[j];
    }
    p--;
  }

 


 /* printf("\n\n");
  i = 0;
  q = 1;
  for(i = 0,p = 1; i < N-1; i++){
    for(j = q; j < N; j++){
      printf("b[%d][%d] = %d     : %d\n",i, j, b[i][j], w+1);
      w++;
    }
    q++;
    }*/
  
  i = 0;
  q = 1;
  r = 1;
  h = 1;
  w = 10000000;
  
  while(1){
    
    if(h == 8){
      break;
    }
    
    for(i = 0,p = 1; i < N-1; i++){
      for(j = q; j < N; j++){
	if(i != 0 && i != h && j != h){ 
	  if(b[0][h] < b[i][j]){
	    r++;
	  }
	}
      }
      
      q++;
    }
    
    if(M >= r){
      if(w > a[h]){
	w = a[h];
      }
    }
    
    h++;
    r = 1;
    q = 1;
  }
  
  if(w == 10000000){
    w = -1;
  }

  printf("%d\n", w);

  
  return 0;
}