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#include <iostream>
#include <vector>
#include <algorithm>
#include <fstream>
#include <map>
#include <set>

using namespace std;

// #define USE_CERR_LOG

#ifdef USE_CERR_LOG
#define LOG if (true) cerr
#else
#define LOG if (false) cerr
#endif

#ifdef USE_FILE_CIN
ifstream input("ora.in");
#define cin input
#endif


size_t bottleCount, diffCount;
vector<int> bottles;

enum InitialCheck {
    ZERO, SOLVABLE, NOT_SOLVABLE
};

InitialCheck isSolvable() {
    set<int> brands;
    size_t i;
    InitialCheck result;

    for (i = 0; i < diffCount; i++) {
        brands.insert(bottles[i]);
    }

    if (brands.size() == diffCount) {
        result = ZERO;
    } else {
        for (; i < bottleCount; i++) {
            brands.insert(bottles[i]);
        }

        result = (brands.size() >= diffCount) ? SOLVABLE : NOT_SOLVABLE;
    }

    return result;
}


long long solve() {
    map<int, int> brandToIdx;
    map<int, int> idxToBrand;
    long long result = 0;

    for (size_t i = 0; i < bottleCount; i++) {
        brandToIdx.insert({bottles[i], i});
    }
    for (auto const& bi : brandToIdx) {
        idxToBrand.insert({bi.second, bi.first});
    }

    auto idxIter = idxToBrand.begin();
    for (size_t i = 0; i < diffCount; i++) {
        LOG << "(" << i << "," << idxIter->first << ":" << idxIter->second << ") ";
        result += idxIter->first - i;
        idxIter ++;
    }

    return result;
}


int main() {
    ios_base::sync_with_stdio(false);
    cin >> bottleCount >> diffCount;
    bottles.resize(bottleCount);

    for (size_t i = 0; i < bottleCount; i++) {
        cin >> bottles[i];
    }

    InitialCheck initialCheck = isSolvable();
    if (initialCheck == SOLVABLE) {
        cout << solve();
    } else {
        cout << (initialCheck == ZERO ? 0 : -1);
    }


    return 0;
}