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#include <algorithm>
#include <cassert>
#include <cctype>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <deque>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>
using namespace std;

// clang-format off
typedef long long LL;
typedef vector<int> VI;

#define REP(i,n) for(int i=0;i<(n);i++)
#define FOR(i,a,b) for(int i=(a);i<=(b);i++)
#define FORD(i,a,b) for(int i=(a);i>=(b);i--)
#define FOREACH(i,c) for(__typeof((c).begin())i = (c).begin();i!=(c).end(); ++i)

//v5
int cond = 1;string to_string(char c){return'\''+string(1,c)+'\'';}string to_string(const string&s){return'"'+s+'"';}string to_string(const char*s){return to_string((string)s);}string to_string(bool b){return(b?"true":"false");}string to_string(std::vector<bool>::reference&r){return to_string((bool)r);}template<typename A,typename B>string to_string(const pair<A,B>&p){return"("+to_string(p.first)+", "+to_string(p.second)+")";}template<typename A>string to_string(A v){int f=1;string r="{";for(auto x:v){if(!f){r+=", ";}f=0;r+=to_string(x);}r+="}";return r;}template<typename A,typename B>string to_string(map<A,B>v){int f=1;string r="{";for(auto x:v){if(!f){r+=", ";}f=0;r+=to_string(x.first)+" -> "+to_string(x.second);}r+="}";return r;}void __db(int i,const vector<string>&s){cerr<<endl;}template<typename T,typename... A>void __db(int i,const vector<string>&s,T t,A... a){if(i)cerr<<", ";cerr<<s[i]<<" = "<<to_string(t);__db(i+1,s,a...);}string __cl(string s){while(!s.empty()&&s[0]==' ')s.erase(0,1);while(!s.empty()&&s.back()==' ')s.pop_back();return s;}vector<string>__vars(string s){vector<string>r;s+=",";string n="";int d=0;for(auto x:s)if(x==','&&d==0){if(!n.empty())r.push_back(__cl(n));n="";}else{n+=x;if(x=='('||x=='[')d++;else if(x==')'||x==']')d--;}return r;}
#define DB(...){if(cond){cerr<<"Line:"<<__LINE__<<" ";__db(0,__vars(#__VA_ARGS__),__VA_ARGS__);}}
// clang-format on

int main() { 
  int n, k;
  LL result = 0;
  scanf("%d%d", &n, &k);
  vector<bool> was(n + 1, 0);
  int taken = 0;
  REP(i, n) {
    int x;
    scanf("%d", &x);
    if (taken < k && was[x] == 0) {
      was[x] = true;
      result += i - taken;
      taken++;
    }
  }
  if (taken != k) {
    printf("-1");
  } else {
    printf("%lld", result);
  }
  return 0; 
}