N=int(input())
S=input()
D={'R':0,'G':1,'B':2}
S=list(map(D.get,S))
pos=[[]for _ in range(3)]
for i,c in enumerate(S):
 pos[c].append(i)
nxt=[N]*N
for l in pos:
 siz=len(l)
 for j in range(siz-1):
  nxt[l[j]]=l[j+1]
dp=[0]*(N+1)
for i in range(N):
 if nxt[i]!=N:
  dp[nxt[i]]=max(dp[nxt[i]],dp[i]+1)
print(N-max(dp))