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#include<cstdio>
#include<iostream>
#include<algorithm>
#include<string>
#include<vector>
#include<unordered_set>

using namespace std;

typedef vector<int> VI;
typedef long long LL;

#define FOR(x, b, e) for(int x=b; x<=(e); ++x)
#define FORD(x, b, e) for(int x=b; x>=(e); --x)
#define REP(x, n) for(int x=0; x<(n); ++x)
#define VAR(v, n) __typeof(n) v = (n)
#define ALL(c) (c).begin(), (c).end()
#define SIZE(x) ((int)(x).size())
#define FOREACH(i, c) for(VAR(i, (c).begin()); i != (c).end(); ++i)
#define PB push_back
#define ST first
#define ND second

int main() {
	ios_base::sync_with_stdio(0);
    int n, k;
    cin >> n >> k;
    VI a(n);
    REP(i, n) cin >> a[i];
    LL res = 0;
    unordered_set<int> placed;
    int j = 1;
    REP(i, k) {
        if(placed.find(a[i]) == placed.end()) {
            placed.insert(a[i]);
        } else {
            j = max(j, i + 1);
            while(j < n && placed.find(a[j]) != placed.end()) j++;
            if(j < n) {
                res += j - i;
                placed.insert(a[j++]);
            } else {
                cout << "-1\n";
                return 0;
            }
        }
    }
    cout << res << '\n';
    return 0;
}