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#include <bits/stdc++.h>
using namespace std;
int n, k, a[500003], t[500003], counter=1, wynik, counter2;
bitset <500002> b;
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
cin >> n >> k;
for(int i=1;i<=n;i++)
{
cin >> a[i];
}
for(int i=1;i<=n;i++)
{
if(b[a[i]]==true)
continue;
else
{
wynik+=i-counter;
counter++;	
}

b[a[i]]=true;
}

for(int i=1;i<=n;i++)
{
	if(b[i]==true)
		counter2++;
}

if(counter2<k)
cout << -1;
else
cout << wynik;

return 0;
}