```ts
export class LruCache<K, V> {
  private readonly capacity: number;
  private readonly cache = new Map<K, { value: V; timestamp: number }>();
  private time = 0;

  constructor(capacity: number) {
    if (typeof capacity !== 'number' || capacity <= 0 || !Number.isInteger(capacity)) {
      throw new Error('Capacity must be a positive integer');
    }
    this.capacity = capacity;
  }

  get(key: K): V | undefined {
    const entry = this.cache.get(key);
    if (entry) {
      entry.timestamp = ++this.time;
      return entry.value;
    }
    return undefined;
  }

  set(key: K, value: V): void {
    if (this.cache.has(key)) {
      this.cache.get(key)!.timestamp = ++this.time;
    } else if (this.cache.size >= this.capacity) {
      const oldestEntry = Array.from(this.cache.entries()).sort((a, b) => a[1].timestamp - b[1].timestamp)[0];
      this.cache.delete(oldestEntry[0]);
    }
    this.cache.set(key, { value, timestamp: ++this.time });
  }

  get size(): number {
    return this.cache.size;
  }
}
```