INT FUNC ModInverse(INT a,b)
  INT t,nt,r,nr,q,tmp

  IF b<0 THEN b=-b FI
  IF a<0 THEN a=b-(-a MOD b) FI
  t=0 nt=1
  r=b nr=a MOD b
  WHILE nr#0
  DO
    q=r/nr
    tmp=nt nt=t-q*nt t=tmp
    tmp=nr nr=r-q*nr r=tmp
  OD
  IF r>1 THEN
    RETURN (-1)
  FI
  IF t<0 THEN
    t==+b
  FI
RETURN (t)

PROC Test(INT a,b)
  INT res

  res=ModInverse(a,b)
  IF res>=0 THEN
    PrintF("%I MODINV %I=%I%E",a,b,res)
  ELSE
    PrintF("%I MODINV %I has no result%E",a,b)
  FI
RETURN

PROC Main()
  Test(42,2017)
  Test(40,1)
  Test(52,-217)
  Test(-486,217)
  Test(40,2018)
RETURN