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#include<bits/stdc++.h>
using namespace std;

int diff = 0;
bool exist[500010];
int res = 0;
bool used[500010];
int t[500010];

int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n, k;
cin >> n >> k;
for (int i = 0; i < n; i++) {
    cin >> t[i];
}
for (int i = 0; i < n; i++) {
    if(!exist[t[i]]) {
        diff++;
        exist[t[i]] = true;
    }
}
if (diff < k) {
    cout << "-1";
    return 0;
}
int poz = 0;
int done = 0;
while (k > 0) {
    while(used[t[poz]]) {
        poz++;
    }
    res += poz - done;
    done++;
    used[t[poz]] = true;
    k--;
}
cout << res;
}