code: def findpk2(self, r1, s1, r2, s2, flag1, flag2):
          
          R1 = self.ec.decompress(r1, flag1)
          R2 = self.ec.decompress(r2, flag2)
  
          rdiff = self.GFn.value(r1 - r2)
  
          return (R1 * s1 - R2 * s2) * (1 / rdiff)
docs: find pubkey Y from 2 different signature on the same message
          sigs: (r1,s1) and (r2,s2)
          returns  (R1*s1-R2*s2)/(r1-r2)