According to the original paper on page 4, $224 \times 224 \times 3$ image is reduced to $112 \times 112 \times 64$ using a filter $7 \times 7$ and stride $2$ after convolution.
- $n \times n = 224 \times 224$
- $f \times f = 7 \times 7$
- stride: $s = 2$
- padding: $p = 0$
The output of the convolution is $(((n+2p-f)/s)+1)$ (according to this), so we have $(n+2p-f)=(224+0-7)=217$, then we divide by the stride, i.e. $217/2=108.5$ (taking the lower value), then we add 1, i.e. $118+1=119$.
How do we get an output image of $112$ now?