# 14241 슬라임 합치기

# 14241
n = str(input())
data = list(map(int, input().split()))

score = 0 
i=0
while i < len(data)-1:
  score = score + data[i]*data[i+1]
  data[i+1] = data[i]+ data[i+1]
  i = i +1 

print(score)