------------------------------------------------AC code------------------------------------------------

#include<cstdio>

using namespace std;

int n, m, val[3] = {0, 3, 4};

int main(){
    scanf("%d%d", &n, &m); n--, m--;
    if(n == 0){
        if(m % 3)   puts("-1");
        else    printf("%d", m/3*2);
    }
    else if(n == 1){
        if(m % 3)   puts("-1");
        else if(m)  printf("%d", m/3*2 + 1);
        else    puts("1");
    }
    else    printf("%d", m/3*2 + n/3*2 + val[m%3] + val[n%3] - 2*((m>=3 && m%3) + (n>=3 && n%3)));
    return 0;
}