{"problem":{"name":"TAKOYAKI FESTIVAL 2019","description":{"content":"It's now the season of TAKOYAKI FESTIVAL! This year, $N$ takoyaki (a ball-shaped food with a piece of octopus inside) will be served. The _deliciousness_ of the $i$\\-th takoyaki is $d_i$. As is common","description_type":"Markdown"},"platform":"AtCoder","limit":{"time_limit":2000,"memory_limit":262144},"difficulty":"None","is_remote":true,"is_sync":true,"sync_url":null,"sign":"abc143_b"},"statements":[{"statement_type":"Markdown","content":"It's now the season of TAKOYAKI FESTIVAL!\nThis year, $N$ takoyaki (a ball-shaped food with a piece of octopus inside) will be served. The _deliciousness_ of the $i$\\-th takoyaki is $d_i$.\nAs is commonly known, when you eat two takoyaki of deliciousness $x$ and $y$ together, you restore $x \\times y$ health points.\nThere are $\\frac{N \\times (N - 1)}{2}$ ways to choose two from the $N$ takoyaki served in the festival. For each of these choices, find the health points restored from eating the two takoyaki, then compute the sum of these $\\frac{N \\times (N - 1)}{2}$ values.\n\n## Constraints\n\n*   All values in input are integers.\n*   $2 \\leq N \\leq 50$\n*   $0 \\leq d_i \\leq 100$\n\n## Input\n\nInput is given from Standard Input in the following format:\n\n$N$\n$d_1$ $d_2$ $...$ $d_N$\n\n[samples]","is_translate":false,"language":"English"}],"meta":{"iden":"abc143_b","tags":[],"sample_group":[["3\n3 1 2","11\n\nThere are three possible choices:\n\n*   Eat the first and second takoyaki. You will restore $3$ health points.\n*   Eat the second and third takoyaki. You will restore $2$ health points.\n*   Eat the first and third takoyaki. You will restore $6$ health points.\n\nThe sum of these values is $11$."],["7\n5 0 7 8 3 3 2","312"]],"created_at":"2026-03-03 11:01:14"}}