IMO 1969 LL MON43

Let p and q be two prime numbers greater than 3. Prove that

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IMO 1969 LL MON43

Origin: MON

Problem

Let p and q be two prime numbers greater than 3. Prove that if their difference is 2n, then for any two integers m and n, the number S = p2m+1 + q2m+1 is divisible by 3.