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#include <cstdio>
#include <vector>

int main() {
    int N, K;
    int result = 0;
    int found = 0;
    std::vector<bool> visited;

    scanf("%d %d", &N, &K);
    visited.resize(N+1, false);

    for (int i=0; i<N; ++i) {
        int x;
        scanf("%d", &x);

        if (visited[x] == false) {
            result += (i-found);
            found++;
        }

        if (found==K) {
            break;
        }

        visited[x] = true;
    }

    if (K==found) {
        printf("%d\n", result);
    } else {
        printf("-1\n");
    }

    return 0;
}