$n$ people sit in a circle. Each of them is either a liar (always lies) or a truthteller (always tells the truth). Every person knows exactly who speaks the truth and who lies. In their turn, each person says 'the person two seats to my left is a truthteller'. It is known that there's at least one liar and at least one truthteller in the circle.

[list=a]
[*] Is it possible that $n=2017?$
[*] Is it possible that $n=5778?$
[/list]
### Part (a): Is it possible that \( n = 2017 \)?

1. **Define the problem in terms of variables:**
   Let \( P_1, P_2, P_3, \ldots, P_{2017} \) be the people sitting in a circle. Define \( a_i \) such that:
   \[
   a_i = \begin{cases} 
   1 & \text{if } P_i \text{ is a truthteller} \\
   0 & \text{if } P_i \text{ is a liar}
   \end{cases}
   \]

2. **Establish the relationship:**
   Each person says "the person two seats to my left is a truthteller." This implies:
   \[
   a_i = a_{i+2} \quad \forall i \in \{1, 2, 3, \ldots, 2017\}
   \]
   where indices are taken modulo 2017.

3. **Analyze the sequence:**
   Since \( a_i = a_{i+2} \), the sequence \( a_1, a_3, a_5, \ldots, a_{2015}, a_{2017} \) must be the same as \( a_2, a_4, a_6, \ldots, a_{2016} \).

4. **Check the periodicity:**
   The sequence \( a_i \) must repeat every 2 steps. Given that 2017 is an odd number, this implies:
   \[
   a_1 = a_3 = a_5 = \cdots = a_{2017} \quad \text{and} \quad a_2 = a_4 = a_6 = \cdots = a_{2016}
   \]
   Since 2017 is odd, the sequence cannot be split evenly into two groups of truthtellers and liars. Therefore, all \( a_i \) must be the same, which contradicts the condition that there is at least one liar and one truthteller.

5. **Conclusion:**
   It is not possible for \( n = 2017 \).

\[
\boxed{\text{No}}
\]

### Part (b): Is it possible that \( n = 5778 \)?

1. **Define the problem in terms of variables:**
   Let \( P_1, P_2, P_3, \ldots, P_{5778} \) be the people sitting in a circle. Define \( a_i \) such that:
   \[
   a_i = \begin{cases} 
   1 & \text{if } P_i \text{ is a truthteller} \\
   0 & \text{if } P_i \text{ is a liar}
   \end{cases}
   \]

2. **Establish the relationship:**
   Each person says "the person two seats to my left is a truthteller." This implies:
   \[
   a_i = a_{i+2} \quad \forall i \in \{1, 2, 3, \ldots, 5778\}
   \]
   where indices are taken modulo 5778.

3. **Analyze the sequence:**
   Since \( a_i = a_{i+2} \), the sequence \( a_1, a_3, a_5, \ldots, a_{5777} \) must be the same as \( a_2, a_4, a_6, \ldots, a_{5778} \).

4. **Check the periodicity:**
   The sequence \( a_i \) must repeat every 2 steps. Given that 5778 is an even number, we can split the sequence into two groups:
   \[
   a_1 = a_3 = a_5 = \cdots = a_{5777} \quad \text{and} \quad a_2 = a_4 = a_6 = \cdots = a_{5778}
   \]
   We can assign all odd-indexed people as truthtellers (\( a_i = 1 \) for odd \( i \)) and all even-indexed people as liars (\( a_i = 0 \) for even \( i \)).

5. **Conclusion:**
   It is possible for \( n = 5778 \).

\[
\boxed{\text{Yes}}
\]