1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
#include <cstdio>
#include <cstring>
#include <cmath>
#include <cassert>
#include <iostream>
#include <algorithm>
#include <iterator>
#include <string>
#include <vector>
#include <queue>
#include <bitset>
#include <utility>
#include <stack>

using namespace std;
typedef long long LL;
typedef pair<int,int> PII;
typedef vector<int> VI;
#define MP make_pair
#define FOR(v,p,k) for(int v=(p);v<=(k);++v)
#define FORD(v,p,k) for(int v=(p);v>=(k);--v)
#define REP(i,n) for(int i=0;i<(n);++i)
#define VAR(v,i) __typeof(i) v=(i)
#define FOREACH(i,c) for(VAR(i,(c).begin());i!=(c).end();++i)
#define PB push_back
#define ST first
#define ND second
#define SIZE(x) (int)x.size()
#define ALL(c) c.begin(),c.end()

#define ODD(x) ((x)%2)
#define EVEN(x) (!(ODD(x)))

int t[500009], dup[500009];
int main() {
    int n, k;
    scanf("%d %d\n", &n, &k);
    REP(i, n) {
        scanf("%d", &t[i]);
    }
    int dup_cnt = 0;
    LL res = 0;
    REP(i, k) {
        int a=t[i];
        res += dup_cnt;
        if (dup[a] > 0) {
            dup_cnt++;
        }
        dup[a]++;
    }
    FOR(i, k, n-1) {
        int a=t[i];
        if (dup_cnt > 0) {
            res += dup_cnt;
        }
        if (dup[a] == 0) {
            dup_cnt--;
            dup[a]++;
        }
    }
    if (dup_cnt > 0) {
        res = -1;
    }
    printf("%lld\n", res);
    return 0;
}