#include <iostream>
#include <unordered_map>

int main() {
    int n, k, x, pos, off;
    long long res = 0LL;
    std::unordered_map<int, int> data;
    std::cin >> n >> k;
    pos = 0;
    off = 0;
    while(pos < n) {
        std::cin >> x;
        auto found = data.find(x);
        if(found == data.end()) {
            data[x] = 0;
        } else {
            if(data.size() < (long unsigned int)k) {
                res += k-pos+off;
                off++;
            }
        }
        data[x] += 1;
        pos++;
    }
    if(data.size() < (long unsigned int)k) {
        res = -1;
    }
    std::cout << res << std::endl;
    return 0;
}