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// Karol Kosinski 2021
#include <bits/stdc++.h>
// #define ENABLE_DEBUG
#define FOR(i,a,b) for(int i=(a),_b=(b);i<_b;++i)
#define FR_(i,a,b) for(int i=(a),_b=(b);i<=_b;++i)
#define FD_(i,b,a) for(int i=(b),_a=(a);i>=_a;--i)
#define ALL(c) (c).begin(),(c).end()
#define SIZE(c) int((c).size())
#define TIE(x...) int x;tie(x)
#define X first
#define Y second
#ifndef ENABLE_DEBUG
#define DEB(k,f,x...)
#else
#define DEB(k,f,x...) {if(k)printf("--------%4d : %s\n",__LINE__,__FUNCTION__);f(x);}
#endif
#define DEBL DEB(1,void,0)
#define DEBF(f,x...) DEB(1,f,x)
#define DEBUG(x...) DEB(0,printf,x)
using namespace std;
using LL = long long;
using ULL = unsigned long long;
using PII = pair<int, int>;
using TIII = tuple<int, int, int>;

constexpr int NX = 500'003;

int A[NX];
bool Counter[NX];
vector<int> Redun;

int main()
{
    int n, k;
    scanf("%d%d", &n, &k);
    FOR(i,0,n) scanf("%d", A + i);
    Redun.reserve(k);
    LL sum = 0;
    
    FOR(i,0,k)
    {
        if ( not Counter[ A[i] ] )
        {
            DEBUG("+ %d\n", i);
            Counter[ A[i] ] = true;
        }
        else Redun.push_back(i);
    }
    reverse( ALL(Redun) );
    int change = SIZE(Redun);
    FOR(i,0,change)
    {
        DEBUG("- %d\n", Redun[i]);
        sum += (k - 1) - i - Redun[i];
    }
    
    if (change > 0) FOR(i,k,n) if ( not Counter[ A[i] ] )
    {
        DEBUG("* %d %d %d\n", i, k, change);
        Counter[ A[i] ] = true;
        sum += i - (k - change);
        -- change;
        if (change == 0) break;
    }
    
    if (change > 0) printf("-1\n");
    else printf("%lld\n", sum);
    return 0;
}