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#include <stdio.h>

int TailBrands[500001];
int N, K;

int main() {
	scanf("%d %d", &N, &K);
	int time = 0;
	int bottleCount = 0;
	for(int i=0; i<K; i++) {
		int brand;
		scanf("%d", &brand);
		if(TailBrands[brand] == 1) {
			time += K - i;
			bottleCount++;
		}
		if(TailBrands[brand] == 0)
			TailBrands[brand] = 1;
	}
	for(int i=K; i<N && bottleCount > 0; i++) {
		int brand;
		scanf("%d", &brand);
		if(TailBrands[brand] == 0) {
			bottleCount--;
			time += i - K;
			TailBrands[brand] = 1;
		}
	}
	if(bottleCount > 0)
		printf("-1");
	else
		printf("%d", time);
	return 0;
}