#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  ;
  int n, k;
  cin >> n >> k;
  vector<int> a(n);
  for (int &x : a) cin >> x;
  bool can = true;
  for (int i = k; i < n; ++i) {
    if (a[i] != a[k - 1]) {
      can = false;
      break;
    }
  }
  if (can) {
    int head = k - 2;
    while (head >= 0 && a[head] == a[k - 1]) head--;
    cout << head + 1;
  } else
    cout << -1;
  return 0;
}
