SOCIAL SECURITY FOR MIGRANT WORKERS - INVALIDITY INSURANCE - CALCULATION OF BENEFITS - AMOUNT OF BENEFIT NOT DEPENDENT ON THE LENGTH OF PERIODS OF INSURANCE - DETERMINATION OF THE THEORETICAL AMOUNT - DISTINCTION BETWEEN PERIODS OF INSURANCE COMPLETED UNDER NATIONAL LEGISLATION AND THOSE COMPLETED BY VIRTUE OF COMMUNITY LEGISLATION - DISTINCTION RESULTING IN A REDUCTION OF THE THEORETICAL AMOUNT BY COMPARISON WITH THE AMOUNT BASED ON NATIONAL LEGISLATION - DISTINCTION NOT PERMISSIBLE ( REGULATION NO 1408/71 OF THE COUNCIL , ART . 46 ( 2 ))