#include <iostream> using namespace std; const int N = 200000; int main() { int n = 0; long long ans = 0; int arr[N] = {0}; cin >> n; for (int i = 0; i < n; i++) { cin >> arr[i]; } long long sum = 0; for (int i = n - 1; i >= 0; i--) { ans += arr[i] * sum; sum += arr[i]; } cout << ans; return 0; }
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