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#include<bits/stdc++.h>
using namespace std;
int B[1000005];
bool W[100005];
int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n, k;
    cin >> n >> k;
    if(k == 1){
        cout << 0;
        return 0;
    }
    for(int i=0; i<n; i++){
        cin >> B[i];
    }
    int t = 0;
    W[B[0]] = true;
    int index = 2;

    for(int i=1; i<k; i++){
        if(W[B[i]]){
            while(index < n and W[B[index]]){
                index++;
            }
            W[B[index]] = true;
            t += (index - i);
        }
        W[B[i]] = true;
    }
    if(index == n){
        cout << -1;
    } else {
        cout << t;
    }
}