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#include<bits/stdc++.h>

using namespace std;

const int INF = 1e6;

int main(){
	int n, k; cin>>n>>k;
	vector<int> a(n); for(int i = 0; i < n; i++)cin>>a[i];
	vector<int> minPos(n, INF);
	for(int i = 0; i < n; i++){
		minPos[a[i]-1] = min(minPos[a[i]-1], i);
	}
	sort(minPos.begin(), minPos.end());
	long long cost = 0;
	bool possible = true;
	for(int i = 0; i < k; i++){
		if(minPos[i] == INF){
			possible = false;
			break;
		}else{
			cost += minPos[i] - i;
		}
	}
	if(!possible){
		cout<<"-1\n";
	}else{
		cout<<cost<<"\n";
	}
}