not primary alignment flag filtering does not work

Hello,

I have a bowtie2 result looking like this;

27823422 reads; of these:   27823422 (100.00%) were unpaired; of these:
2464773 (8.86%) aligned 0 times
19083986 (68.59%) aligned exactly 1 time
6274663 (22.55%) aligned >1 times
91.14% overall alignment rate

I am trying to count the uniquely mapped reads, and in this case I think the value should be 19083986 since it should be aligned once.

I have tried these commands;

samtools view -h -F 256 -c input.bam 
samtools view -h -F 0x100 -c input.bam

But they are both giving the total (27823422) number. Why could it be like that?

Thank you in advance.