```ts
class LruCache<K, V> {
  private readonly capacity: number;
  private size: number = 0;
  private cache: Map<K, V>;
  private usageOrder: K[] = [];

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

  get(key: K): V | undefined {
    if (this.cache.has(key)) {
      const value = this.cache.get(key);
      this.usageOrder = this.usageOrder.filter(k => k !== key);
      this.usageOrder.push(key);
      return value;
    }
    return undefined;
  }

  set(key: K, value: V): void {
    if (this.cache.has(key)) {
      this.usageOrder = this.usageOrder.filter(k => k !== key);
    } else {
      this.size++;
    }
    this.cache.set(key, value);
    this.usageOrder.push(key);

    if (this.size > this.capacity) {
      const oldestKey = this.usageOrder.shift();
      if (oldestKey !== undefined) {
        this.cache.delete(oldestKey);
        this.size--;
      }
    }
  }

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