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)