#include <stdio.h>
using ll=long long;
const int C=1200001;

int count[C], a[C], position[C];
int main(){
    int i, n, k, j=0;
    ll counter=0, res=0;

    scanf ("%d %d", &n, &k);
    for (i=0; i<n; i++) scanf ("%d", &a[i]);
    for (i=0; i<n; i++){
        if (count[a[i]] == 0){
            count[a[i]] = 1;
            counter++;
            position[i]=k;
            continue;
        }
        else position[i] = counter;
    }
    if (counter < k){
        printf ("-1\n");
        return 0;
    }
    for (i=0; i<n; i++){
        if (position[i]<k) res += (k-position[i]); //O tyle przesunąć frajera do przodu
    }
    printf ("%lld\n", res);

return 0;}