654 B | 654 chars
import sys
from math import sqrt, pi, sin, factorial, ceil, floor
BLANK = " "
inp = input
# inp = lambda : sys.stdin.readline()[:-1].strip()
mii = lambda x = BLANK : [*map(int,inp().split(x))]
mfi = lambda x = BLANK : [*map(float,inp().split(x))]
ii = lambda : int(inp())
fi = lambda : float(inp())
p = print
def solve():
n = ii()
s = input()
crt = 1
ans = 0
for i in range(1, n):
if s[i] == s[i - 1]:
crt += 1
else:
if s[i - 1] == 'a' and crt > 1:
ans += crt
crt = 1
if s[-1] == 'a' and crt > 1:
ans += crt
p(ans)
if __name__ == "__main__":
tc = 1
for t in range(1, tc+1):
ret = solve()