bign multi(bign a,int b){
bign c;
int carry=0;
for(int i=0;i<a.len;i++){
int tmp=a.d[i]*b+carry;
c.d[c.len++]=tmp%10;
carry=tmp/10;
}
while(carry!=0){
c.d[c.len++]=carry%10;
carry/=10;
}
return c;
}