#include <bits/stdc++.h>

int bubble(std::vector<int>& xs) {
  int n = xs.size();
  int res = 0;
  for(int i = 1; i < n; ++i) {
    for(int j = n - 1; j >= i; --j) {
      if( xs[j-1] > xs[j] ) {
        res += 1;
        std::swap(xs[j-1], xs[j]);
      }
    }
  }
  return res;
}

bool sorted(std::vector<int>& xs) {
  int n = xs.size();
  for(int i = 1; i < n; ++i) {
    if( xs[i-1] > xs[i] ) return false;
  }
  return true;
}

int main() {
  int n; scanf("%d", &n);
  int k; scanf("%d", &k);
  int xs[1024];
  for(int i = 0; i < n; ++i) {
    scanf("%d", &xs[i]);
  }

  std::vector<int> array[1024];
  for(int i = 0; i < n; ++i) {
    array[i % k].push_back(xs[i]);
  }
  int res = 0;
  for(int i = 0; i < k; ++i) {
    res += bubble(array[i]);
  }
  std::vector<int> ys;
  for(int i = 0; i < n; ++i) {
    ys.push_back(array[i%k][i/k]);
  }

  if( not sorted(ys) ) {
    printf("-1\n");
    return 0;
  }
  printf("%d\n", res);
  
  
  
  return 0;
}