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#include<bits/stdc++.h>
 
using namespace std;
 
using ll = long long;
using vi = vector<int>;
using pi = pair<int, int>;
using vpi = vector<pi>;
#define FOR(x, b, e) for(int x = b; x <= (e); ++x)
#define FORD(x, b, e) for(int x = b; x >= (e); --x)
#define REP(x, n) for(int x = 0; x < (n); ++x)
#define ALL(c) (c).begin(), (c).end()
#define ALLD(c) (c).rbegin(), (c).rend()
#define SIZE(x) ((int)(x).size())
#define pb push_back
#define pp emplace_back
#define st first
#define nd second
#define sq(a) (a)*(a)
const int inf = 1000000001;
const double eps = 1e-9;
bool fleq(double a, double b){
	return abs(a - b) < eps;
}

bool t[500100];
bool t2[500100];
int n, k;
ll ans;
ll dif;
int main(){
	ios_base::sync_with_stdio(0);
	cin.tie(0);
    cin >> n >> k;
    vi a(n);
    REP(i, n){
        cin >> a[i];
        t[a[i]]=true;
    }
    FOR(i, 1, n){
        if(t[i])dif++;
        t[i] = false;
    }
    if(dif < k){
        cout << "-1\n";
        return 0;
    }
    int j = k;
    REP(i, k){
        t[a[i]] = true;
    }
    REP(i, k){
        if(t2[a[i]]){
            while(t[a[j]])j++;
            a[i] = a[j];
            ans += j - i;
            j++;
        }
        t[a[i]] = true;
        t2[a[i]] = true;
    }
    cout << ans << '\n';
	return 0;
}