import sys

sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()
dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
# inf = 18446744073709551615
inf = 4294967295
# md = 10**9+7
md = 998244353

def rindex(aa, v):
    for i in range(len(aa))[::-1]:
        if aa[i] == v: return i
    return -1

def solve(aa, si):
    res = -1
    if len(aa)-1 < si: return inf
    l = aa.index(1)
    r = rindex(aa, 1)
    if l==r==si:
        return 3

    if l < si:
        res += si-l
        for i in range(l, si): aa[i] ^= 1
        si = l

    aa[si] ^= 1
    for i in range(si, r):
        if i == r and aa[i] == 0: break
        res += 1
        aa[i] ^= 1
        if aa[i]:
            aa[i] ^= 1
            aa[i+1] ^= 1
            res += 2
    if aa[r]: res += 1
    return res

si = II()-1
aa = [(c == "#")*1 for c in SI()]
n = len(aa)

if sum(aa) == 0:
    print(0)
    exit()

ans = min(solve(aa[:], si), solve(aa[::-1], n-1-si))
print(ans)