#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
using ll=long long;
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
template<class T> bool chmax(T &a, T b){if (a < b){a = b;return true;} else return false;}
template<class T> bool chmin(T &a, T b){if (a > b){a = b;return true;} else return false;}

template <class T>
struct FenwickTree{
    int n;
    vector<T> bit;
    FenwickTree(int N){
        bit.resize(N+1);
        n = N;
    }
    T sum(int i){
        i++;
        T s = 0;
        while (i > 0){
            s += bit[i];
            i -= i & -i;
        }
        return s;
    }
    T sum(int l, int r){
        if (l >= r) return 0;
        return sum(r-1) - sum(l-1);
    }
    void add(int i, T x){
        i++;
        while (i <= n){
            bit[i] += x;
            i += i & -i;
        }
    }
};

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int N;
    ll M;
    cin>>N>>M;
    vector<int>P(N);
    FenwickTree<int>fw(N);
    ll cnt=0;
    for(int i=0;i<N;i++){
        cin>>P[i];
        P[i]--;
        cnt+=fw.sum(P[i],N);
        fw.add(P[i],1);
    }
    if(cnt==0)cout<<0<<"\n";
    else if(cnt%2==0){
        if(M%2==0)cout<<max(cnt,M)<<"\n";
        else cout<<max(cnt,2*M)<<"\n";
    }else{
        if(M%2==0)cout<<-1<<"\n";
        else cout<<max(cnt,M)<<"\n";
    }
}