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#include <cstdio>
#include <vector>
#include <list>

using namespace std;

int main() {
    int n,k, moves = 0;
    scanf("%d %d", &n, &k);
    vector<int> c(n,0);
    list<int> stack;
    if (k == 1) {
        printf("0\n");
        return 0;
    }
    for (int i = 0; i < n; ++i) {
        int b;
        scanf("%d", &b);
        if (i < k) {
            if (c[b-1] == 1) {
                stack.push_back(i);
            }
            c[b-1] = 1;
        }
        else {
            if (c[b-1] == 0 && !stack.empty()) {
                int f = stack.front();
                stack.pop_front();
                moves += (i-f);
                c[b-1] = 1;
            }
            if (stack.empty()) {
                break;
            }
        }
    }
    if (!stack.empty()) {
        printf("%d\n", -1);
    }
    else {
        printf("%d\n", moves);
    }
    return 0;
}