#include <iostream>
#include <vector>
#include <cmath>
#include <iomanip>
#include <array>
#include <tuple>
#include <algorithm>
#include <map>
#include <stack>
#include <queue>
#include <set>
#include <list>
#include <cstdio>
#include <limits.h>
#include <string.h>

using namespace std;
using ull = unsigned long long;
using ll = long long;
using lli = long long int;
using ld = long double;
using pa = pair<int,int>;
#define SORT(v, n) sort(v, v+n);
#define VSORT(v) sort(v.begin(), v.end());
#define GRESORT(v) sort(v.begin(),v.end(),greater<int>())
#define REVERSE(v) reverse(v.begin(),v.end())
#define overlap(v) v.erase(unique(v.begin(),v.end()),v.end())
#define debug(x) cout << #x << ": " << x << endl
#define INF 999999999

//4近傍
int dy[]={0, 0, 1, -1};
int dx[]={1, -1, 0, 0};

//8近傍
//int dy[]={0,0,1,-1,1,1,-1,-1};
//int dx[]={1,-1,0,0,1,-1,1,-1};

int n,k;
vector<int> a(1001),b(1001);


void input(){
    cin >> n >> k;
    for(int i = 0; i < n; i++) cin >> a[i];
    for(int i = 0; i < n; i++) b[i] = a[i];
    VSORT(b);
    overlap(b);
}

void solve(){
    int count = 0;
    int i = 0;
    while(i < n-k){
        if(a[i] > a[i+k]){
            int val = a[i];
            a[i] = a[i+k];
            a[i+k] = val;
            i = 0;
            count++;
        }else i++;
    }
    for(i = 0; i < n; i++){
        if(a[i] != b[i+1]){
            count = -1;
            break;
        }
    }
    cout << count << endl;
}

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

    input();
    solve();

    return 0;
}