## https://yukicoder.me/problems/no/1606


def main():
    N = int(input())
    A = list(map(int, input().split()))

    intervals = []
    prev = 2
    for a in A:
        if a != 2:
            if len(intervals) == 0:
                array = [0, 0]
                array[a] = 1
                intervals.append(array)
            else:
                if prev != 2:
                    intervals[-1][a] += 1
                else:
                    array = [0, 0]
                    array[a] = 1
                    intervals.append(array)
        prev = a

    value = 0    
    a1_value = 0
    for a0, a1 in intervals:
        value += a0 + a1
        a1_value += a1
    if a1_value == 0 or ( value - a1_value) == 0:
        print(0)
        return

        
    dp = [float("inf") for _ in range(value + 1)]
    dp[0] = 0

    for a0, a1 in intervals:
        new_dp = [float("inf") for _ in range(value + 1)]
        total = a0 + a1
        for i in range(value + 1):
            # そのまま
            new_dp[i] = min(new_dp[i], dp[i])

            # 加える
            if i + total <= value:
                new_dp[i + total] = min(new_dp[i + total], dp[i] + a0)
        dp = new_dp

    answer = float("inf")
    for total in range(value + 1):
        if dp[total] == float("inf"):
            continue

        a0 = dp[total]
        a1 = total - a0

        # 相手側
        a = a1_value - a1
        if a0 == a:
            answer = min(answer, a0)
    if answer == float("inf"):
        print(-1)
    else:
        print(answer)




                


if __name__ == "__main__":
    main()