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#include <cstdio>
#include <cmath>
#include <iostream>
#include <algorithm>
#include <queue>
#include <list>
#include <vector>
#include <map>
#include <set>
#include <string>
#include <cstring>
#include <cassert>
#include <limits.h>
#include <unordered_set>

using namespace std;

typedef long long LL;
typedef long double LD;

typedef vector<int> VI;
typedef pair<int,int> PII;
#define REP(i,n) for(int i=0;i<(n);i++)
#define FOR(i,a,b) for(VAR(i,a);i<=(b);++i)
#define FORD(i,a,b) for(VAR(i,a);i>=(b);--i)
#define FORE(a,b) for(VAR(a,(b).begin());a!=(b).end();++a)
#define VAR(a,b) __typeof(b) a=(b)
#define SIZE(a) ((int)((a).size()))
#define ALL(x) (x).begin(),(x).end()
#define CLR(x,a) memset(x,a,sizeof(x))
#define ZERO(x) memset(x,0,sizeof(x))
#define S(t,i) scanf("%" ## t, &i)
#define SI(i) scanf("%d", &i)


int n, m;
unordered_set<int> nums;
long long int result;

int main() {
    //ios_base::sync_with_stdio(0);
    
    SI(n);
    SI(m);
    
    
    int num;
    int num_different = 0;
    
    FOR(i, 1, n) {
        SI(num);
        if (nums.find(num) == nums.end()) {
//            cout<<num<<" "<<i - num_different - 1<<"\n";
            nums.insert(num);
            result += i - num_different - 1;
            num_different++;
        }
        if (num_different == m) {
            break;
        }
    }
    
    cout << (num_different == m ? result : -1 ) << "\n";
    
    return 0;
}