#include <bits/stdc++.h>

int ri() {
	int n;
	scanf("%d", &n);
	return n;
}
int64_t rs64() {
	int64_t n;
	scanf("%" SCNd64, &n);
	return n;
}
int main() {
	int n = ri(), k = ri();
	if (k > n) {
		puts("-1");
		return 0;
	}
	printf("%d\n", k - 1);
	return 0;
}