#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n, a, b;
  cin >> n >> a >> b;
  int arr[n];
  for (int i = 0; i < n; i++) {
    cin >> arr[i];
  }
  sort(arr, arr + n);
  int pety = arr[n - a];
  int vasy = arr[b - 1];
  printf("%d\n", abs(pety - vasy));
}
