str = raw_input();
a,b = str.split(' ');
a,b = int(a),int(b);
flag = 0;

arr = []

for i in range(0,a):
	x = raw_input();
	arr.append(x);

for j in range(0,26):
	cnt = 0;
	for i in range(0,a):
		if arr[i].find(chr(97+j)) != -1:
			cnt+=1;
	if cnt == a:
		flag+=1;
		

if flag < b: print ":-(";
else : print ":-)";