n=int(input())
a=list(map(int,input().split()))

ans=[0]*n
cnt = 1

for i in range(1,n):
    if a[i-1] + 1 == a[i]:
        ans[i] = ans[i-1] + cnt
        cnt += 1
    else:
        ans[i] = ans[i-1]
        cnt = 1

print(ans[-1])