#include <bits/stdc++.h>
using namespace std;
void printprecise(double l, long long int precision) {
  std::cout << std::fixed;
  std::cout << std::setprecision(precision);
  std::cout << l;
}
long long int modular(long long int x, long long int n, long long int m) {
  if (n == 0)
    return 1;
  else if (n % 2 == 0)
    return modular((x * x) % m, n / 2, m);
  else
    return (x * modular((x * x) % m, (n - 1) / 2, m)) % m;
}
long long int gcd(long long int a, long long int b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
void initialize(long long int ar[], long long int n) {
  for (long long int x = 0; x < n; x++) ar[x] = x;
}
void printarray(long long int ar[], long long int n) {
  for (long long int x = 0; x < n; x++) cout << ar[x] << " ";
  cout << endl;
}
void printvector(vector<long long int> v) {
  for (long long int x = 0; x < v.size(); x++) {
    cout << v[x] << " ";
  }
  cout << endl;
}
long long int root(long long int i, long long int ar[]) {
  while (ar[i] != i) {
    ar[i] = ar[ar[i]];
    i = ar[i];
  }
  return i;
}
void unione(long long int a, long long int b, long long int ar[]) {
  long long int p = root(a, ar);
  long long int q = root(b, ar);
  ar[p] = ar[q];
}
void bfs(long long int n, long long int vis[], long long int level[],
         vector<long long int> v[]) {
  list<long long int> g;
  g.push_back(n);
  vis[n] = 1;
  level[n] = 0;
  while (!g.empty()) {
    long long int a = g.front();
    g.pop_front();
    for (long long int x = 0; x < v[a].size(); x++) {
      if (vis[v[a][x]] == 0) {
        g.push_back(v[a][x]);
        vis[v[a][x]] = 1;
        level[v[a][x]] = level[a] + 1;
      }
    }
  }
}
void dfs(long long int n, vector<long long int> v[], long long int vis[]) {
  vis[n] = 1;
  for (long long int x = 0; x < v[n].size(); x++) {
    if (vis[v[n][x]] == 0) {
      dfs(v[n][x], v, vis);
    }
  }
}
void computelpsarray(string s, long long int lps[]) {
  long long int i = 1;
  long long int j = 0;
  lps[0] = 0;
  while (i < s.length()) {
    if (s[i] == s[j]) {
      lps[i] = j + 1;
      i++;
      j++;
    } else {
      if (j == 0) {
        lps[i] = 0;
        i++;
      } else {
        j = lps[j - 1];
      }
    }
  }
}
void getZarr(string str, long long int Z[]) {
  long long int n = str.length();
  long long int L, R, k;
  L = R = 0;
  Z[0] = 0;
  for (long long int i = 1; i < n; ++i) {
    if (i > R) {
      L = R = i;
      while (R < n && str[R - L] == str[R]) R++;
      Z[i] = R - L;
      R--;
    } else {
      k = i - L;
      if (Z[k] < R - i + 1)
        Z[i] = Z[k];
      else {
        L = i;
        while (R < n && str[R - L] == str[R]) R++;
        Z[i] = R - L;
        R--;
      }
    }
  }
}
int main() {
  long long int n, d;
  cin >> n >> d;
  long long int ar1[n], ar2[n];
  for (long long int x = 0; x < n; x++) {
    cin >> ar1[x];
  }
  for (long long int x = 0; x < n; x++) {
    cin >> ar2[x];
  }
  long long int r = ar1[d - 1] + ar2[0];
  ar1[d - 1] = r;
  long long int i = -1;
  for (long long int x = 0; x < n; x++) {
    if (ar1[x] <= r) {
      i = x;
      break;
    }
  }
  for (long long int x = 0; x < i; x++) {
    ar1[x] = ar1[x] + ar2[x + 1];
  }
  long long int ie = n - 1;
  for (long long int x = i; x < n; x++) {
    if (x != d - 1) {
      ar1[x] = ar1[x] + ar2[ie--];
    }
  }
  long long int c = 0;
  for (long long int x = 0; x < n; x++) {
    if (ar1[x] > r) {
      c++;
    }
  }
  cout << c + 1;
}
