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#include "bits/stdc++.h"
 
using namespace std;
 
template<typename _T> void _debug(const char *s, _T x){ cerr << s << " = " << x << "\n";}
template<typename _T, typename... R> void _debug(const char *s, _T x, R... r){ while(*s != ',') cerr << *s++; cerr << " = " << x << ", "; _debug(s + 1, r...);}
 
#define debug(...)	_debug(#__VA_ARGS__,  __VA_ARGS__);
 
#define sz(s)	int32_t(s.size())
 
using ll = long long;
using ld = long double;
 
// #define int long long  

const int N = 5e5 + 5;

bool vis[N];

int pos[N];

int32_t main(){
	ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    int n, k;   cin >> n >> k;
    vector < int > arr(n), vec;
    for(int i = 0; i < n; i++){
        cin >> arr[i];
    }
    for(int i = 0; i < n; i++){
        if(vis[arr[i]]) continue;
        vis[arr[i]] = true;
        vec.push_back(arr[i]);
        if(sz(vec) == k)    break;
    }
    if(sz(vec) != k){
        cout << "-1\n";   return 0;
    }
    int ans = 0;
    for(int i = 0, j = 0; i < k; i++, j++){
        while(vec[i] != arr[j]){
            j++;
        }
        ans += j - i;  
    }
    cout << ans << "\n";
}