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#include <bits/stdc++.h>
using namespace std;

typedef long long LL;
typedef long double LD;
typedef pair<int, int> PII;
typedef pair<LL, LL> PLL;

#define all(x) (x).begin(), (x).end()
#define sz(x) (int)(x).size()

template<typename _T> inline void _DBG(const char *s, _T x) { cerr << s << " = " << x << "\n"; }
template<typename _T, typename... args> void _DBG(const char *s, _T x, args... a) { while(*s != ',') cerr << *s++; cerr << " = " << x << ','; _DBG(s + 1, a...); }

#ifdef LOCAL
#define _upgrade ios_base::sync_with_stdio(0);
#define DBG(...) _DBG(#__VA_ARGS__, __VA_ARGS__)
#else
#define _upgrade ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define DBG(...) (__VA_ARGS__)
#define cerr if(0) cout
#endif

// ********************** CODE ********************** //

const int N = 5e5 + 7;

int n, k, T[N];
bool was[N];

int main()
{
    _upgrade
    
    cin >> n >> k;
    for (int i = 1; i <= n; i++)
        cin >> T[i];
    
    int distinct_left = k;
    LL i = 1, ans = 0;
    while (distinct_left > 0 && i <= n)
    {
        if (!was[T[i]])
        {
            distinct_left--;
            was[T[i]] = true;
        }
        else
        {
            ans -= i;
        }
        i++;
    }

    if (distinct_left > 0)
    {
        cout << -1 << "\n";
        return 0;
    }

    for (int j = i - 1; j > k; j--)
        ans += j;
   
    cout << ans << "\n";
    return 0;
}