#include <stdio.h>

int main(void) {
    int L,K,M,i,answer=-1;
    scanf("%d", &L);
    scanf("%d", &K);
    M = L / 2;
    if(M-K>0) {
        for(i=L; i>0; i-=K) {
            answer++;
        }
    } else {
        answer = 0;
    }
    printf("%d",answer);
    return 0;
}