C语言想在while的条件里判断a既不等于1也不等于2时进入循环 但是2不行 1也可以判断为什么

int main()
{
    int a;
    float b;
    printf("Enter the unit of Weight([1]-lbs, [2]-kg):");
    scanf("%d",&a);


    while(a!= 1&&2)
    {
        printf("Invalid Number!\n");
        printf("Enter the unit of Weight([1]-lbs, [2]-kg):");
        scanf("%d",&a);
    }

a!=1 && a!=2