903D - Almost Difference - CodeForces Solution

n = int(input())
arr = [int(i) for i in input().split()]
xrr = dict()
ans = 0
s = arr[0]
c = 1
xrr.update({arr[0]:1})
for i in range(1,n):
    if(arr[i]-1 not in xrr):
        xrr.update({arr[i]-1:0})
    if(arr[i]+1 not in xrr):
        xrr.update({arr[i]+1:0})
    if(arr[i] not in xrr):
        xrr.update({arr[i]:0})
    p = s-xrr[arr[i]-1]*(arr[i]-1)-xrr[arr[i]+1]*(arr[i]+1)
    q = c-xrr[arr[i]-1]-xrr[arr[i]+1]
    ans += (q*arr[i])-p
    s += arr[i]
    c += 1
    xrr[arr[i]] += 1
print(ans)