The difficulty is a part of the block header called "bits". It is the 72nd to 75th bytes of the block header and looks like this in hex:

bits = 0x76270618

Remember this is little-endian! Actual big-endian version is 0x18062776 Take the first byte as the “exponent” and the other three bytes as the “coefficient”

Exponent = 0x18 = 24

Coefficient = 0x062776

Plug into this formula:

Coefficient * 2**(8*(Exponent - 3)) = 0x0000000000000000062776000000000000000000000000000000000000000000

And that's how you can calculate difficulty from bits.