a=int(input())

b=list(input())

c=0#R의 개수

d=0#B의 개수

for i in range(0,len(b)):

if b[i]=='R':

c+=1

else:

d+=1

if b[0]=='B':

for i in range(0,len(b)):

if b[i]=='R':

break

min_x=len(b)-c-i

if b[0]=='R':

for i in range(0,len(b)):

if b[i]=='B':

break

min_x=len(b)-d-i

if b[a-1]=='B':

for i in range(len(b)-1, -1, -1):

if b[i]=='R':

break

min_x=min(min_x,i-c)

if b[a-1]=='R':

for i in range(len(b)-1,-1,-1):

if b[i]=='B':

break

min_x=min(min_x,i-d)

print(min_x)