def main():
    import sys
    input = sys.stdin.read().split()
    ptr = 0
    N = int(input[ptr])
    ptr += 1
    M = int(input[ptr])
    ptr += 1

    d_out = [0] * (N + 1)
    d_in = [0] * (N + 1)

    for _ in range(M):
        U = int(input[ptr])
        ptr += 1
        V = int(input[ptr])
        ptr += 1
        d_out[U] += 1
        d_in[V] += 1

    delta = []
    sum_delta = 0
    sum_abs = 0
    for u in range(1, N + 1):
        d = d_out[u] - d_in[u]
        delta.append(d)
        sum_delta += d
        sum_abs += abs(d)

    if sum_delta != 0:
        print(-1)
        return

    if sum_abs == 0:
        print(0)
        return
    elif sum_abs == 2:
        # 检查是否有恰好两个顶点的delta为+1和-1
        count_plus = 0
        count_minus = 0
        for d in delta:
            if d == 1:
                count_plus += 1
            elif d == -1:
                count_minus += 1
            elif d != 0:
                # 存在其他值，无法满足条件
                print(0)
                return
        if count_plus == 1 and count_minus == 1:
            print(0)
        else:
            print(-1)
        return
    else:
        if sum_abs % 2 != 0:
            print(-1)
            return
        else:
            add = (sum_abs - 2) // 2
            print(add)
            return

if __name__ == "__main__":
    main()