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#include <cstdio>
#include <stdio.h>
#include <iostream>
#include <string>
#include <stdint.h>
#include <unordered_set>
#include <algorithm>


using namespace std;


static const int32_t MAX_SIZE = 500000;

static const int32_t MAX_LABEL = 500000;


static int32_t iN, iK;

static int64_t iResult;

static int32_t aShelf[MAX_SIZE];

static int8_t aUsed[MAX_LABEL + 2] = { 0, };


static void ReadData()
{
    cin >> iN >> iK;

    for (int32_t i = 0; i < iN; ++i)
    {
        cin >> aShelf[i];
    }
}


static void Solve()
{
    int32_t iDst = 0;

    iResult = 0;

    for (int32_t iSrc = 0; iSrc < iN; ++iSrc)
    {
        int32_t iCurr = aShelf[iSrc];

        if (aUsed[iCurr] != 0)
        {
            continue;
        }

        aUsed[iCurr] = 1;

        iResult += (iSrc - iDst++);

        if (iDst >= iK)
            break;
    }

    if (iDst < iK)
        iResult = -1;
}


int main(int argc, char * argv[])
{
    ReadData();

    Solve();

    cout << iResult;

    return 0;
}