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#include <iostream>
using namespace std;
int tab[500005] {};
bool b[500005] {};
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);

int n,k,u=0;
long long unsigned cz=0;
cin>>n>>k;
for (int i=0;i<n;i++)
{cin>>tab[i];}

for (int i=0;i<n;i++)
{if (!b[tab[i]])
{u++;
b[tab[i]]=1;
cz=cz+i-u+1;}
if (u==k) 
{cout<<cz;
return 0;}
}
cout<<"-1";

return 0;
}