C语言 头结点链表输出不了

#include<stdio.h>
#include<stdlib.h>
typedef struct None{
    int val;
    struct None *next;
}None;

main(){
    int flag=1,a;
    None *head,*q,*tab;
    head=(None*)malloc(sizeof(None));
    head->next=NULL;
    tab=head;
    while(flag){
        scanf("%d",&a);
        q=(None*)malloc(sizeof(None));
        if(a=-1){
            break;
        }
        q->val=a;
        tab->next=q;
        tab=q;
    }
    tab->next=NULL;
    
    head=head->next;
    while(head!=NULL){
        printf("%d",head->val);
        head=head->next;
    }
}

哭了，又找不出问题

if(a=-1)
改为if(a==-1)