#include <bits/stdc++.h>
using namespace std;

#define rep(i,n) for (int i=0;i<(n);i++)
#define rep2(i,a,b) for (int i=(a);i<(b);i++)
#define rrep(i,n) for (int i=(n)-1;i>=0;i--)
#define rrep2(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define all(a) (a).begin(),(a).end()

typedef long long ll;
typedef pair<int, int> P;


const int MAX_N = 1 << 18;

// [1, n]
// BIT is initialized by zeros
// !!initialize the size ""N""!!
int bit[MAX_N + 1], N, K;

void init() {
    fill(bit, bit + N, 0);
}

// get sum in [1, i]
// sum{[i, j]} = sum{[1, j]} - sum{[1, i-1]}
int sum(int i) {
    int s = 0;
    while (i > 0) {
        s += bit[i];
        i -= i & -i;
    }
    return s;
}

// add x to bit[i]
void add(int i, int x) {
    while (i <= N) {
        bit[i] += x;
        i += i & -i;
    }
}

signed main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);

    cin >> N >> K;
    vector<int> a(N);
    vector<P> b(N);
    rep(i, N) {
        cin >> a[i];
        b[i] = P(a[i], i);
    }

    sort(all(b));
    rep(i, N) {
        if (b[i].second % K != i % K) {
            cout << -1 << endl;
            return 0;
        }
        a[b[i].second] = i + 1; // compress a[] to 1, 2, ...
    }

    reverse(all(a));
    ll ans = 0;
    // modグループごとに反転数を加算
    rep(k, K) {
        init();
        for (int i = k; i < N; i += K) {
            ans += sum(a[i] - 1);
            add(a[i], 1);
        }
    }


    cout << ans << endl;
}