{"raw_statement":[{"iden":"problem statement","content":"You are given an integer $N$. Build an undirected graph with $N$ vertices with indices $1$ to $N$ that satisfies the following two conditions:\n\n*   The graph is simple and connected.\n*   There exists an integer $S$ such that, for every vertex, the sum of the indices of the vertices adjacent to that vertex is $S$.\n\nIt can be proved that at least one such graph exists under the constraints of this problem."},{"iden":"constraints","content":"*   All values in input are integers.\n*   $3 \\leq N \\leq 100$"},{"iden":"input","content":"Input is given from Standard Input in the following format:\n\n$N$"},{"iden":"sample input 1","content":"3"},{"iden":"sample output 1","content":"2\n1 3\n2 3\n\n*   For every vertex, the sum of the indices of the vertices adjacent to that vertex is $3$."}],"translated_statement":null,"sample_group":[],"show_order":["default"],"formal_statement":null,"simple_statement":null,"has_page_source":true}