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#include<bits/stdc++.h>
#define ST first
#define ND second
#define ll long long
#define ld long double
#define imie(...) " [" << #__VA_ARGS__ ": " << (__VA_ARGS__) << "] "
using namespace std;
 
const ll  INF = 1e9 + 9;
const ll  MOD = 1e9 + 7;
const long long LINF = (ll)1e18 + 3;

int main(){
	ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
    
    int n,k; cin >> n >> k;
    vector<int> cnt(n+1), tab(n+1);
    int different = 0, endInd = -1;
    for(int i = 0; i < n; i++){
        cin >> tab[i];
        if(cnt[tab[i]] == 0) different++;
        cnt[tab[i]]++;
        if(different == k){
            endInd = i;
            break;
        }
    }
    if(endInd == -1){
        cout << -1 << "\n";
        return 0;
    }
    ll ans = 0;
    int subtract = 0;
    for(int i = endInd; i >= 0; i--){
        if(cnt[tab[i]] > 1){
            ans += (ll)(endInd - subtract - i); 
            subtract++;
            cnt[tab[i]]--;
        }
    }
    cout << ans << "\n";
	return 0;
}