dp=[0,0,0]
n=int(input())
s=input()
p2=0
for i in range(n):
    dp2=[0,0,0]
    for j in range(3):
        if ord(s[i])==(ord('A')+j):
            dp2[j]=min(dp[j],dp[(j+1)%3]+p2,dp[(j+2)%3]+p2)
        else:
            dp2[j]=min(dp[j]+p2*2+1,dp[ord(s[i])-ord('A')]+p2*2+1,dp[3-j-(ord(s[i])-ord('A'))]+p2+1)
    dp=dp2
    p2=p2*2+1
print(dp[0]%1000000007)