#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, d, i, j = 0, x, y, ind, rank;
  cin >> n >> d;
  vector<long long> a;
  long long b[n - 1];
  for (i = 1; i <= n; i++) {
    cin >> x;
    if (i != d) {
      a.push_back(x);
    } else {
      y = x;
    }
  }
  x = y;
  cin >> y;
  x = x + y;
  for (i = 0; i < n - 1; i++) {
    cin >> b[i];
  }
  reverse(b, b + n - 1);
  for (i = 0; i < n - 1; i++) {
    if (a[i] <= x) {
      break;
    }
  }
  ind = i;
  j = 0;
  rank = ind + 1;
  while (i < n - 1) {
    if (a[i] + b[j] == x) {
      j++;
    } else if (a[i] + b[j] > x) {
      rank++;
    } else {
      j++;
    }
    i++;
  }
  cout << rank;
}
