Let A,B C {1,..., n}. For m G Z, let r AtB {m) be the cardinality of the set of ordered pairs (a, b) 6 A x B such that a + b = m. For t ^ 1, denote by (A + B) t the set of the elements m for which r^si'm) ^ t. In this paper we prove that for any subsets A,B C {l,...,n} such that |v4| + | 5 | ^ (4n + 4 t -3 ) / 3 , the sumset (A + B) t contains a block of consecutive integers with the length at least \A\ + \B\ -2t +1, and that (a) for any two subsets A and B of {1,..., n} such that \A\ + \B\ ^ (4n)/3, there exists an arithmetic progression of length n in A + B; (b) for any 2 $C r < (An -l)/3, there exist two subsets A and B of {1,... , n} with \A\ + \B\ = r such that any arithmetic progression in A + B has the length at most (2n -l)/3 + 1.