import sys
n = int(sys.stdin.readline().strip())
data = list(map(int, sys.stdin.readline().strip().split()))
dp=[0]*n
start = n - 1
for i in range(n - 1, -1, -1):
    if i + data[i] >= start:
        dp[i]=max(dp[i],dp[start]+data[i])
        start = i
if start == 0:
    print(dp[0])
else:
    print(-1)