```typescript
export class LruCache<K, V> {
  private capacity: number;
  private cache: Map<K, { value: V; timestamp: number }>;

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

  get(key: K): V | undefined {
    const entry = this.cache.get(key);
    if (entry) {
      entry.timestamp = Date.now(); // Mark as recently used
      return entry.value;
    }
    return undefined;
  }

  set(key: K, value: V): void {
    if (this.cache.has(key)) {
      this.cache.set(key, { value, timestamp: Date.now() });
    } else {
      this.cache.set(key, { value, timestamp: Date.now() });
      if (this.cache.size > this.capacity) {
        const oldestEntry = Array.from(this.cache.entries()).reduce((oldest, [k, v]) => {
          return v.timestamp < oldest.timestamp ? [k, v] : oldest;
        }, [null, null])[0];
        if (oldestEntry) {
          this.cache.delete(oldestEntry[0]);
        }
      }
    }
  }

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