A compound of the formula I: ** Formula ** or a salt thereof, wherein: R 1 is alkyl of 1 to 6 carbon atoms, alkenyl of 2 to 6 carbon atoms, alkynyl of 2 to 6 carbon atoms, cycloalkyl of 3 to 6 carbon atoms, cycloalkenyl of 5 to 6 carbon atoms or cycloalkyl of 3 to 6 carbon atoms-alkyl of 1 to 4 carbon atoms； wherein this alkyl of 1 to 6 carbon atoms, alkenyl of 2 to 6 carbon atoms, alkynyl of 2 to 6 carbon atoms or cycloalkyl of 3 to 6 carbon atoms-alkyl of 1 to 4 carbon atoms may be substituted once or more than once by halogen, hydroxyl, or alkoxy of 1 to 4 carbon atoms； and wherein the cycloalkyl of 3 to 6 carbon atoms or cycloalkenyl of 5 to 6 carbon atoms may be substituted once or more than once by halogen, hydroxyl, alkyl of 1 to 4 carbon atoms, halo-alkyl of 1 at 4 carbon atoms, hydroxyalkyl of 1 to 4 carbon atoms or alkoxy of 1 to 4 carbon atoms； m is 1 or 2； n is 0, 1, 2, 3 or 4； each R2 is independently halogen, hydroxyl, amino, cyano, nitro, alkyl of 1 to 6 carbon atoms, halo-alkyl of 1 to 6 carbon atoms, hydroxy-alkyl of 1 to 6 carbon atoms, alkoxy of 1 to 4 carbon-alkyl atoms of 1 to 6 carbon atoms, amino-alkyl of 1 to 6 carbon atoms, alkyl of 1 to 4 carbon atoms-amino-alkyl of 1 to 6 carbon atoms, di- (alkyl of 1 at 4 carbon atoms) -amino alkyl of 1 to 6 carbon atoms, alkoxy of 1 to 6 carbon atoms, halo-alkoxy of 1 to 6 carbon atoms, alkyl of 1 to 6 carbon atoms-amino, di- (alkyl of 1 to 6 carbon atoms) -amino, alkenyl of 2 to 6 carbon atoms, halo-alkenyl of 2 to 6 carbon atoms, alkynyl of 2 to 6 carbon atoms or halo-alkynyl of 2 to 6 atoms carbon； or cycloalkyl of 3 to 6 carbon atoms, wherein a carbon atom can be replaced by an oxygen atom, wherein the cycloalkyl of 3 to 6 carbon atoms can be attached directly to methylene or by means of an alkylene of 1 at 2 carbon atoms, and wherein the cycloalkyl of 3 to 6 carbon atoms may be substituted once or more than once by halogen； or two R2 in the same carbon atom together with that carbon atom, a cycloalk
