  The measurement of cosmic ray energy spectra, in particular for individual
species, is an essential approach in finding their origin. Locating the "knees"
of the spectra is an important part of the approach and has yet to be achieved.
Here we report a measurement of the mixed Hydrogen and Helium spectrum using
the combination of the ARGO-YBJ experiment and of a prototype Cherenkov
telescope for the LHAASO experiment. A knee feature at 640+/-87 TeV, with a
clear steepening of the spectrum, is observed. This gives fundamental inputs to
galactic cosmic ray acceleration models.
