#include<bits/stdc++.h>
using namespace std;
using ll=long long;
using pii=pair<int,int>;

#define all(a) a.begin(),a.end()
#define pb push_back
#define sz(a) ((int)a.size())

const int N=200005,mod=998244353;
int add(int x, int y){x+=y; if(x>=mod) x-=mod; return x;}
int sub(int x, int y){x-=y; if(x<0) x+=mod; return x;}
int mul(int x, int y){return ((ll)x)*y%mod;}
int Pow(int x, ll y=mod-2){int res=1; for(; y; x=mul(x,x),y>>=1) if(y&1) res=mul(res,x); return res;}
int fac[N],inv[N],ifac[N];
inline int C(int n, int m){if(m<0||m>n) return 0; return mul(fac[n],mul(ifac[m],ifac[n-m]));}
void init_comb(){
    fac[0]=inv[1]=ifac[0]=1;
    for(int i=1; i<N; ++i) fac[i]=mul(fac[i-1],i);
    for(int i=2; i<N; ++i) inv[i]=mul(inv[mod%i],mod-mod/i);
    for(int i=1; i<N; ++i) ifac[i]=mul(ifac[i-1],inv[i]);
}
int bit[N];
void modify(int p, int x){for(++p; p<N; p+=p&-p) bit[p]+=x;}
int sum(int r){
    int res=0;
    for(++r; r; r-=r&-r) res+=bit[r];
    return res;
}

int n,a[N];
ll m;

signed main(){
    ios_base::sync_with_stdio(0),cin.tie(0);
    cin >> n >> m;
    for(int i=0; i<n; ++i) cin >> a[i];
    ll res=0;
    for(int i=n-1; i>=0; --i){
        res+=sum(a[i]);
        modify(a[i],1);
    }
    ll x=(res+m-1)/m*m;
    if((x-res)%2==0) cout << x << "\n";
    else if((x+m-res)%2==0) cout << x+m << "\n";
    else cout << "-1\n";
}