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#include<bits/stdc++.h>
using namespace std;
using LL=long long;
#define FOR(i,l,r) for(int i=(l);i<=(r);++i)
#define REP(i,n) FOR(i,0,(n)-1)
#define ssize(x) int(x.size())
template<class A,class B>auto&operator<<(ostream&o,pair<A,B>p){return o<<'('<<p.first<<", "<<p.second<<')';}
template<class T>auto operator<<(ostream&o,T x)->decltype(x.end(),o){o<<'{';int i=0;for(auto e:x)o<<(", ")+2*!i++<<e;return o<<'}';}
#ifdef DEBUG
#define debug(x...) cerr<<"["#x"]: ",[](auto...$){((cerr<<$<<"; "),...)<<'\n';}(x)
#else
#define debug(...) {}
#endif

int main() {
	cin.tie(0)->sync_with_stdio(0);

	int n, k;
	cin >> n >> k;
	debug(n, k);
	vector<int> v(n);
	REP(i, n) {
		cin >> v[i];
		--v[i];
	}
	debug(v);
	LL ans = 0;
	int x = 0;
	vector<bool> visited(n);
	REP(i, k) {
		while (x < n && visited[v[x]]) {
			++x;
		}
		if (x == n) {
			cout << "-1\n";
			return 0;
		}
		ans += x - i;
		visited[v[x]] = true;
	}
	cout << ans << '\n';
}