#include <iostream>
#include <vector>

typedef long long ll;
using namespace std;

int main() {
    ios_base::sync_with_stdio(0); cin.tie(0);
    int n, k;
    cin>>n>>k;
    vector<int> a(n);
    for(int& v : a) {
        cin>>v; --v;
    }

    vector<bool> used(n, false);
    int tail, head;

    for(int i = 0; i < k; ++i) {
        if(used[a[i]]) {
            tail = i;
            head = i + 1;
            break;
        }
        used[a[i]] = true;
    }
    
    ll total = 0;
    for(; tail < k; ++tail) {
        while(head < n && used[a[head]]) {
            ++head;
        }
        if(head >= n) {
            cout<<-1;
            return 0;
        }
        used[a[head]] = true;
        total += head - tail;
    }

    cout<<total;
}