#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long n;
  cin >> n;
  long long k;
  cin >> k;
  long long arr[n + 1];
  for (long long i = 1; i <= n; i++) {
    cin >> arr[i];
  }
  set<long long> s;
  for (long long i = k; i <= n; i++) {
    s.insert(arr[i]);
  }
  if (s.size() != 1) {
    cout << -1 << endl;
  } else {
    long long index = k;
    while (index >= 1 && arr[index] == arr[k]) {
      index--;
    }
    index++;
    cout << index - 1 << endl;
  }
}
