N=int(raw_input())
S=raw_input()
cnt=[0 for i in range(30)]

for s in S:
    cnt[ord(s)-ord('a')]+=1
ans=0
m=N

z=cnt[ord('z')-ord('a')]
y=cnt[ord('y')-ord('a')]
x_v=0
for X in range(ord('x'),ord('u'),-1):
    x_v+=cnt[X-ord('a')]
u=cnt[ord('u')-ord('a')]
t_l=0
for X in range(ord('t'),ord('k'),-1):
    t_l+=cnt[X-ord('a')]
k=cnt[ord('k')-ord('a')]
j=cnt[ord('j')-ord('a')]
i=cnt[ord('i')-ord('a')]
h_a=0
for X in range(ord('a'),ord('i')):
    h_a+=cnt[X-ord('a')]

yukia=min(y,u,k,i,h_a)
ans+=yukia
y,u,k,i,h_a=y-yukia,u-yukia,k-yukia,i-yukia,h_a-yukia
#print y,u,k,i
yukii=min(y,u,k,i/2)
ans+=yukii
y,u,k,i=y-yukii,u-yukii,k-yukii,i-yukii-yukii
yukj=min(y,u,k,j)
ans+=yukj
y,u,k=y-yukj,u-yukj,k-yukj
yukk=min(y,u,k/2)
ans+=yukk
y,u,k=y-yukk,u-yukk,k-yukk-yukk
yut=min(y,u,t_l)
ans+=yut
y,u=y-yut,u-yut
yuu=min(y,u/2)
ans+=yuu
y,u=y-yuu,u-yuu-yuu
yx=min(y,x_v)
ans+=yx
y-=yx
yy=y/2
ans+=yy
y-=2*yy
ans+=z
#print yukia,yukii,yukj,yukk,yut,yuu,yx,yy,z
print ans