import sys

int1 = lambda x: int(x) - 1

# input = lambda: sys.stdin.buffer.readline()
input = lambda: sys.stdin.readline().rstrip()
ii = lambda: int(input())
i1 = lambda: int1(input())
mi = lambda: map(int, input().split())
mi1 = lambda: map(int1, input().split())
li = lambda: list(mi())
li1 = lambda: list(mi1())
lli = lambda n: [li() for _ in range(n)]

INF = float("inf")
mod = int(1e9 + 7)
# mod = 998244353

n = ii()
s = input()
a = [i for i in range(n)[::-1] if s[i] == "#"]


ng = 0
ok = int(1e18)
while ok - ng > 1:
    mid = ok + ng >> 1
    mn = INF
    pre = INF
    for x in a:
        mn = min(mn, x + mid, pre - 2)
        pre = min(x + mid, pre - 2)
    if mn >= n:
        ok = mid
    else:
        ng = mid
print(ok)