n = int(input())
arr = list(map(int, input().split()))
max_reach = 0
for i in range(n):
    if i > max_reach:
        print('false')
        break
    max_reach =max(max_reach,i+ arr[i])
else:
    print('true' if max_reach >= n-1 else 'false')
#动态规划效率不够