//   _|                                                _|
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//   _|_|      _|    _|  _|_|_|_|  _|    _|  _|_|      _|_|      _|_|_|_|  _|_|_|_|
//   _|  _|    _|    _|  _|        _|    _|      _|_|  _|  _|    _|        _|
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//                   _|            _|
//               _|_|              _|
#include <iostream>

using namespace std;

// #define DEBUG
#ifdef DEBUG
#define dassert(x) assert(x);
#define df(...) printf(__VA_ARGS__)
#else
#define dassert(x)
#define df(...)
#endif

#define x first
#define y second
#define mp make_pair
#define pb push_back
#define ir(x, a, b) ((a) <= (x) && (x) <= (b))
#define vec vector
#define sz(x) (ll)x.size()
#define foru(i, n) for (int i = 0; (i) < (n); ++(i))
#define all(x) (x).begin(), (x).end()
#define qed return 0

typedef int64_t ll;
typedef pair<ll, ll> pll;

int read() {
    int n = 0; bool b = 0; char c = getchar();
    for (; !ir(c, '0', '9'); c = getchar()) b = (c == '-');
    for (; ir(c, '0', '9'); c = getchar()) n = 10*n + (c-'0');
    if (b) return -n;
    return n;
}

const int N = 5e5+10;
int a[N];
bool t[N];
int main() {
    df("debug mode\n");
#ifndef DEBUG
    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#endif
    int n, k;
    cin >> n >> k;
    foru (i, n) {
        cin >> a[i];
    }
    int l = 0, s = 0;
    foru (i, n) {
        if (!t[a[i]]) {
            s += i-l;
            ++l;
            t[a[i]] = 1;
        }
        if (l == k) {
            cout << s << "\n";
            return 0;
        }
    }
    cout << "-1\n";
    return 0;
}