# gemini-3.5-flash-lite

def solve():
    import sys
    input = sys.stdin.read
    data = input().split()
    
    if not data:
        return
        
    N = int(data[0])
    S = data[1]
    
    # 1-indexed で扱いやすくするために先頭にダミー文字を入れる
    S = "_" + S
    
    left_dist = [float('inf')] * (N + 1)
    right_dist = [float('inf')] * (N + 1)
    
    # 1. 左から走査
    last_person = -float('inf')
    for i in range(1, N + 1):
        if S[i] == '#':
            last_person = i
        left_dist[i] = i - last_person
        
    # 2. 右から走査
    last_person = float('inf')
    for i in range(N, 0, -1):
        if S[i] == '#':
            last_person = i
        right_dist[i] = last_person - i
        
    # 3 & 4. 各空席について最も近い人までの距離の最大値を求める
    max_min_dist = -1
    best_seat = -1
    
    for i in range(1, N + 1):
        if S[i] == '.':
            # 左右の近い方の距離
            current_min_dist = min(left_dist[i], right_dist[i])
            
            # 最大値を更新（同点の場合はより左側の席を優先するため、厳密に大きい場合のみ更新）
            if current_min_dist > max_min_dist:
                max_min_dist = current_min_dist
                best_seat = i
                
    print(best_seat)

if __name__ == '__main__':
    solve()