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#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;


bool B[500004];
int T[500004];
int main() {
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    int n,k;
    cin >> n >> k;
    for (int i=0;i<=n;i++) B[i] = false;
    for (int i=0;i<n;i++) cin >> T[i];
    long long result = 0;
    int i;
    for (i=0;i<n && k>0;i++)
    {
        int a = T[i];
        if (B[a]) break;
        k--;
        B[a]=true;
    }
    int differ = i+1;
    while (differ < n && k>0)
    {
        if (B[T[differ]]) {differ++; continue;}
        result += differ - i;
        B[T[differ]] = true;
        i++;
        differ++;
        k--;
    }
    if (k!=0) result = -1;
    cout << result << '\n';
}