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#include <stdio.h>

int set[1000000];
int a[1000000];

int main()
{
    int n, k;
    
    scanf("%d%d", &n, &k);
    for (int i = 0; i < n; i++)
        scanf("%d", &a[i]);
    
    int look=0;
    int pos=0;
    long long result=0;
    
    while (pos<k && look<n)
    {
        if (set[a[look]] == 0)
        {
            set[a[look]] = 1;
            look++;
            pos++;
            result += look - pos;
        }
        else look++;
    }
    
    if (pos == k)
        printf("%lld\n", result);
    else
        printf("-1\n");
    
    return 0;
}