  The energy spectrum of cosmic Hydrogen and Helium nuclei has been measured,
below the so-called "knee", by using a hybrid experiment with a wide
field-of-view Cherenkov telescope and the Resistive Plate Chamber (RPC) array
of the ARGO-YBJ experiment at 4300 m above sea level. The Hydrogen and Helium
nuclei have been well separated from other cosmic ray components by using a
multi-parameter technique. A highly uniform energy resolution of about 25% is
achieved throughout the whole energy range (100 TeV - 700 TeV). The observed
energy spectrum is compatible with a single power law with index
gamma=-2.63+/-0.06.
