  The modified Gottfried sum rule multiplied by 3/2 can be interpreted as the
one to measure the mean $I_3$ of the [(quark) - (antiquark)] in the proton .
Based on this interpretation we find the sum rule which can be understood as
the one to measure the mean charge of the light sea quarks (u,d,s) in the
proton, and show that it takes the value 0.23 for the proton and 0.34 for the
neutron .
