C   18/10/85 510181700  MEMBER NAME  AINVMS   (FORT)        FORTRAN77
C
      SUBROUTINE AINVMS(AM0,AM1,PIN0,PIN1,TOTMAS,PCMS)
C
C       INPUT : AM0,1 -- MASS OF PARTICLE 0,1
C             : PIN0,1-- MOM. OF PARTICLE 0,1
C       OUTPUT: TOTMAS-- INVARIANT MASS
C             : PCMS  -- MOMENTUM IN CENTER OF MASS SYSTEM
C
C    GET INV.MASS OF TWO PARTICLE SYSTEM    T.KAJITA, 18-OCT-85
C
      DIMENSION PIN0(3), PIN1(3)
C
      PXTOT=PIN0(1)+PIN1(1)
      PYTOT=PIN0(2)+PIN1(2)
      PZTOT=PIN0(3)+PIN1(3)
      E0 = SQRT ( PIN0(1)**2 + PIN0(2)**2 + PIN0(3)**2 + AM0**2 )
      E1 = SQRT ( PIN1(1)**2 + PIN1(2)**2 + PIN1(3)**2 + AM1**2 )
      ETOT=E0+E1
C
      PTOT = SQRT ( PXTOT**2 + PYTOT**2 + PZTOT**2 )
      TTMAS=ETOT**2 - PTOT**2
      IF(TTMAS.LT.0.) TTMAS=0.
      TOTMAS=SQRT(TTMAS)
C
      PCMS = SQRT( (TOTMAS**2-(AM0+AM1)**2)*(TOTMAS**2-(AM0-AM1)**2)
     &              /(4.*TOTMAS**2) )
C
C
      RETURN
      END