#include <bits/stdc++.h>   using namespace std;   int v[2000];   int a, b, n, res;   int main() {     scanf( %d %d %d , &n, &a, &b);     for (int i = 0; i < n; i++) scanf( %d , &v[i]);     sort(v, v + n);     int res = v[b] - v[b - 1];     cout << res << endl;   }   