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#include<cstdio>
#include<algorithm>
#include<vector>

int occ[600*1000];

int main() {
    int n, k;
    scanf("%i%i", &n, &k);
    
    std::fill(occ, occ+600*1000, -1);
    
    int tmp;
    for (int i = 0; i < n; ++i) {
        scanf("%i", &tmp);
        tmp--;
        occ[tmp] = (occ[tmp] == -1) ? i : occ[tmp];
    }
    
    std::vector<int> tab;
    for (int i = 0; i < n; ++i) {
        if (occ[i] != -1)
            tab.push_back(occ[i]);
    }
    
    std::sort(tab.begin(), tab.end());
    
    if (tab.size() < k)
        printf("-1\n");
    else {
        long long res = 0LL;
        for (int i = 0; i < k; ++i)
            res += tab[i] - i;
        printf("%lli\n", res);
    }
   
    return 0;
}