Educational Codeforces Round 34 (Rated for Div. 2) D. Almost Difference（高精度）

题目链接：http://codeforces.com/contest/903/problem/D

题目不难，按每个元素算贡献就好了，就是会爆long long，学习一下py的写法

代码：

from collections import Counter

n = input()
a = map(int, raw_input().split())
s = 0
c = Counter()
for i in xrange(n):
s += c[a[i] + 1] - c[a[i] - 1] + a[i] * (2 * i + 1 - n)
c[a[i]] += 1
print(s)