#define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES

#include <bits/stdc++.h>
using namespace std;
using i8  = int8_t;  using u8  = uint8_t;
using i16 = int16_t; using u16 = uint16_t;
using i32 = int32_t; using u32 = uint32_t;
using i64 = int64_t; using u64 = uint64_t;
using f32 = float_t; using f64 = double_t;
using usize = size_t;using str = string;
template <class T> using vec = vector<T>;
#define times(n, i) for (i32 i = 0; i < (n); i++)
#define range(n, m, i) for (i32 i = (n); i < (m); i++)
#define upto(n, m, i) for (i32 i = (n); i <= (m); i++)
#define downto(n, m, i) for (i32 i = (n); i >= (m); i--)
#define foreach(xs, x) for (auto &x : (xs))
#define all(xs) (xs).begin(), (xs).end()
#define sortall(xs) sort(all(xs))
#define reverseall(xs) reverse(all(xs))
#define uniqueall(xs) (xs).erase(unique(all(xs)), (xs).end())
#define maximum(xs) *max_element(all(xs))
#define minimum(xs) *min_element(all(xs))
const i64 MOD = 1000000007;

i32 main()
{
    i32 n, k;
    cin >> n >> k;
    vec<i32> a(n);
    vec<i32> z(n);
    times(n, i) {
        cin >> a[i];
        z[i] = a[i];
    }
    sortall(z);
    i32 cnt = 0;
    while (z != a) {
        bool b = false;
        range(0, n-k-1, i) {
            if (a[i] < a[i+k])
                continue;
            i32 t = a[i];
            a[i] = a[i+k];
            a[i+k] = t;
            cnt++;
            b = true;
        }
        if (!b)
            break;
    }
    cout << (z == a ? cnt : -1) << endl;
    return 0;
}