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/* -----------------------
Autor: Tomasz Boguslawski
-------------------------- */
#include<cstdio>
#include<cstdlib>
#include<iostream>
#include<fstream>
#include<iomanip>
#include<string>
#include<sstream>
#include<cstring>
#include<map>
#include<vector>
#include<set>
#include<queue>
#include<algorithm>
#include <fstream>
#include<math.h>

#define LL long long
#define FOR(x, b, e) for(LL x = b; x <= (e); x++)
#define FORD(x, b, e) for(LL x = b; x >= (e); x--)
#define VAR(v, n) __typeof(n) v = (n)
#define ALL(c) (c).begin(), (c).end()
#define FOREACH(i, c) for(VAR(i, (c).begin()); i != (c).end(); ++i)
#define DEBUG if (debug)
#define MIN(a,b) ((a>b)?b:a)
#define MAX(a,b) ((a>b)?a:b)


using namespace std;

LL a[501000];

/// MAIN
int main(int argc, char* argv[])
{
    // magic formula, which makes streams work faster:
	ios_base::sync_with_stdio(0);

	LL n; LL k;
	cin >> n; cin >> k;
	FOR(i,1,n) cin >> a[i];
	LL pom;
	set<LL> already;
	LL cost = 0;
	LL s = 0;
	FOR(j,1,k)
	{
	    // po wykonaniu petli oranzady na pozycjach 1..j beda rozne
	    if (already.count(a[j])>0)
        {
            // element juz byl, trzeba poszukac czegos na wymiane
            if (s<j+1) s=j+1;
            while ((s<=n)&&(already.count(a[s])>0)) s++;
            if (s>n) { cout << "-1\n"; return 0; };
            cost+=(s-j);
            pom=a[s]; a[s]=a[j]; a[j]=pom;
        };
        already.insert(a[j]);
	};
	cout << cost << '\n';
    return 0;
};