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#include<bits/stdc++.h>

using namespace std;

#define PB push_back
#define MP make_pair
#define F first
#define S second
#define ALL(x) (x).begin(),(x).end()
#define SIZE(x) (int)(x).size()
#define CLEAR(tab) memset(tab, 0, sizeof(tab))
#define REP(x, n) for(int x = 0; x < (n); x++)
#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 VAR(v, n) __typeof(n) v = (n)
#define FOREACH(i, c) for(VAR(i, (c).begin()); i != (c).end(); ++i)
#define DEBUG(fmt, ...) fprintf(stderr, fmt, ##__VA_ARGS__)

typedef long long int LL;
typedef pair<int,int> PII;
typedef vector<int> VI;

const int MXN = 1000010;
const int C = 262144;
const int INF = 1000000001;
const int MOD = 1000000007;

int n, k;
int a[MXN];
int taken[MXN];
int b[MXN];

void test() {
	scanf("%d %d", &n, &k);
	for(int i = 1; i <= k; i++)
		scanf("%d", &a[i]);
	int l = n - k;
	LL res = 0;
	for(int i = 1; i <= l; i++)
		scanf("%d", &b[i]);
	int cnt = 0;
	for(int i = 1; i <= k; i++) {
		if(taken[a[i]]) {
			res += k - i;
			cnt++;
		}
		else
			taken[a[i]] = 1;
	}
	for(int i = 1; i <= l; i++) {
		if(cnt > 0 && !taken[b[i]]) {
			cnt--;
			res += i;
			taken[b[i]] = 1;
		}
	}

	if(cnt == 0)
		printf("%lld", res);
	else
		printf("-1");
}

int main() {
	int te = 1;
//	scanf("%d", &te);
	while(te--)
		test();
	return 0;
}