3
rjnflsbpxqivrcdjptj
vpojopbwbwbswdu
rydkiwnugwddcgcrng
 1
z
 30
b
u
p
n
f
m
t
i
j
k
p
h
a
c
w
z
z
p
t
d
o
w
u
q
l
y
i
o
a
u
 5
splt
ohqykk
xqpz
knojbur
pmfm
 2
hk
obsp
 5
zzxpfk
abcdefghijklmnopqrst
uvwxz
srgkjaskldfkln
vgnsdfdgfh
 2
rxscdzkkezud
wjehahqgouqvjienq
 5
ojdfhi
abcdefghijklmnopqrst
uvwxyz
ddfhdfhlasjt
qqq
 1
epkotfpkkrhhmuipmtdk
 10
kpmwcdoysw
gtpr
kuzoxmiixxbl
crgqtuo
jhbplhpklrgwnaugdf
zuxdaat
aycv
qwghrkqwkobrgevsjrk
tdxgc
lxyzgcmbzulcst
 5
dfijdfhi
abcdefghijklmnopqrst
uvwxy
kopsdfgiopjipw
jty
 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
waiphwcqrrinr
o
qiqehzmgsjdoqd
kjexeesevrlowxhghq
judikhzkj
z
xo
lsdzypkfqro
sshgcxsky
gecntpcmoojfwp
svmytmcfhc
jrsrvsvbaiumlmkptn
s
wpcsovfjlyspviflk
ktvyzvddgllht
szahigtmklglrcocbo
znligfxkgxzkcfeu
liryvzmqwhr
xgrxkgiehxztv
etrjxdczppafly
jrdgajschhwsci
doxnxbjwptnimjmsuijx
zciwkbvrhgsjhrr
ehh
bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
x
itaubd
xcn
v
x
i
ci
aqs
zbi
q
cvdu
a
jos
qy
fjf
nayke
ge
s
q
qwad
gvz
o
ubn
nzoc
h
wnjwjrf
t
bq
bw
 1
b

a
 a
 e
 a
 a
 y
 b
 aa
 a
 ab
 z
 e
 b
 aa
 l
 a

l = ['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','x','y','z','aa','ab','ac','ad','ae','af','ag','ah','ai','aj','ak','al','am','an','ao','ap','aq','ar','as','at','au','av','aw','ax','ax','ay','az','ba','bb','bc','bd','be','bf','bg','bh','bi','bj','bk','bl','bm','bn','bo','bp','bq','br','bs','bt','bu','bv','bw','bx','bx','by','bz','ca','cb','cc','cd','ce','cf','cg','ch','ci','cj','ck','cl','cm','cn','co','cp','cq','cr','cs','ct','cu','cv','cw','cx','cx','cy','cz','da','db','dc','dd','de','df','dg','dh','di','dj','dk','dl','dm','dn','do','dp','dq','dr','ds','dt','du','dv','dw','dx','dx','dy','dz','ea','eb','ec','ed','ee','ef','eg','eh','ei','ej','ek','el','em','en','eo','ep','eq','er','es','et','eu','ev','ew','ex','ex','ey','ez','fa','fb','fc','fd','fe','ff','fg','fh','fi','fj','fk','fl','fm','fn','fo','fp','fq','fr','fs','ft','fu','fv','fw','fx','fx','fy','fz','ga','gb','gc','gd','ge','gf','gg','gh','gi','gj','gk','gl','gm','gn','go','gp','gq','gr','gs','gt','gu','gv','gw','gx','gx','gy','gz','ha','hb','hc','hd','he','hf','hg','hh','hi','hj','hk','hl','hm','hn','ho','hp','hq','hr','hs','ht','hu','hv','hw','hx','hx','hy','hz','ia','ib','ic','id','ie','if','ig','ih','ii','ij','ik','il','im','in','io','ip','iq','ir','is','it','iu','iv','iw','ix','ix','iy','iz','ja','jb','jc','jd','je','jf','jg','jh','ji','jj','jk','jl','jm','jn','jo','jp','jq','jr','js','jt','ju','jv','jw','jx','jx','jy','jz','ka','kb','kc','kd','ke','kf','kg','kh','ki','kj','kk','kl','km','kn','ko','kp','kq','kr','ks','kt','ku','kv','kw','kx','kx','ky','kz','la','lb','lc','ld','le','lf','lg','lh','li','lj','lk','ll','lm','ln','lo','lp','lq','lr','ls','lt','lu','lv','lw','lx','lx','ly','lz','ma','mb','mc','md','me','mf','mg','mh','mi','mj','mk','ml','mm','mn','mo','mp','mq','mr','ms','mt','mu','mv','mw','mx','mx','my','mz','na','nb','nc','nd','ne','nf','ng','nh','ni','nj','nk','nl','nm','nn','no','np','nq','nr','ns','nt','nu','nv','nw','nx','nx','ny','nz','oa','ob','oc','od','oe','of','og','oh','oi','oj','ok','ol','om','on','oo','op','oq','or','os','ot','ou','ov','ow','ox','ox','oy','oz','pa','pb','pc','pd','pe','pf','pg','ph','pi','pj','pk','pl','pm','pn','po','pp','pq','pr','ps','pt','pu','pv','pw','px','px','py','pz','qa','qb','qc','qd','qe','qf','qg','qh','qi','qj','qk','ql','qm','qn','qo','qp','qq','qr','qs','qt','qu','qv','qw','qx','qx','qy','qz','ra','rb','rc','rd','re','rf','rg','rh','ri','rj','rk','rl','rm','rn','ro','rp','rq','rr','rs','rt','ru','rv','rw','rx','rx','ry','rz','sa','sb','sc','sd','se','sf','sg','sh','si','sj','sk','sl','sm','sn','so','sp','sq','sr','ss','st','su','sv','sw','sx','sx','sy','sz','ta','tb','tc','td','te','tf','tg','th','ti','tj','tk','tl','tm','tn','to','tp','tq','tr','ts','tt','tu','tv','tw','tx','tx','ty','tz','ua','ub','uc','ud','ue','uf','ug','uh','ui','uj','uk','ul','um','un','uo','up','uq','ur','us','ut','uu','uv','uw','ux','ux','uy','uz','va','vb','vc','vd','ve','vf','vg','vh','vi','vj','vk','vl','vm','vn','vo','vp','vq','vr','vs','vt','vu','vv','vw','vx','vx','vy','vz','wa','wb','wc','wd','we','wf','wg','wh','wi','wj','wk','wl','wm','wn','wo','wp','wq','wr','ws','wt','wu','wv','ww','wx','wx','wy','wz','xa','xb','xc','xd','xe','xf','xg','xh','xi','xj','xk','xl','xm','xn','xo','xp','xq','xr','xs','xt','xu','xv','xw','xx','xx','xy','xz','xa','xb','xc','xd','xe','xf','xg','xh','xi','xj','xk','xl','xm','xn','xo','xp','xq','xr','xs','xt','xu','xv','xw','xx','xx','xy','xz','ya','yb','yc','yd','ye','yf','yg','yh','yi','yj','yk','yl','ym','yn','yo','yp','yq','yr','ys','yt','yu','yv','yw','yx','yx','yy','yz','za','zb','zc','zd','ze','zf','zg','zh','zi','zj','zk','zl','zm','zn','zo','zp','zq','zr','zs','zt','zu','zv','zw','zx','zx','zy','zz']
n = input()
s = []
for i in range(n):
	tmp = raw_input()
	s.append(tmp)
for item in l:
	found = False
	for _str_ in s:
		if _str_.find(item) >= 0:
			found = True
	if not found:
		print item
		break
