PAT/PATBasic/1049.c

#include <stdio.h>

int main()
{
	int N;
	double ai, sum = 0;

	scanf("%d", &N);
	for (int i = 0; i < N; i++) {
		scanf("%lf", &ai);
		/* ai is put at the beginning to avoid overflow */
		sum += ai * (i + 1) * (N - i);
	}
	printf("%.2lf", sum);

	return 0;
}