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#include <iostream>


#ifdef TEST_MODE
#include <fstream>
#endif

using namespace std;

const int kMaxOranges = 500000;

typedef long long ll;

int main() {
#ifdef TEST_MODE
    std::ifstream in("1.in"); std::cin.rdbuf(in.rdbuf());
#endif
    std::ios_base::sync_with_stdio(false);
    int bottels[kMaxOranges];
    int numberOfTypes[kMaxOranges+1];
    int n, k;
    cin >> n >> k;
    for ( int i = 0; i < n; ++i ) {
        numberOfTypes[i] = 0;
    }
    int boundIdx = k - 1;
    for ( int i = 0; i < n; ++i ) {
        cin >> bottels[i];
        numberOfTypes[bottels[i]]++;
        if ( numberOfTypes[bottels[i]] > 1 ) {
            if ( i <= boundIdx ) {
                boundIdx++;
            }
        }
    }   
    if ( boundIdx  >=  n ) {
        cout << -1 << endl;
    } else {
        for ( int i = boundIdx + 1; i < n; ++i ) {
            numberOfTypes[bottels[i]]--;
        }
        ll result = 0;
        for ( int i = boundIdx - 1; i >= 0;  --i ) {
            if ( numberOfTypes[bottels[i]] > 1 ) {
                numberOfTypes[bottels[i]]--;
                result += boundIdx - i;
                boundIdx--;
            }
        }
        cout << result << endl;
    }
    return 0;
}