using System;
using System.Collections.Generic;
using System.Linq;
class MainClass {
static void Main() {
int[] nums = {2, 7, 9, 3, 1};
int maxValue = 0;
int prevValue = 0;
int prevPrevValue = 0;
for(var i=0; i < nums.Length; i++){
maxValue = Math.Max(prevValue, prevPrevValue + nums[i]);
prevPrevValue = prevValue;
prevValue = maxValue;
}
Console.WriteLine(maxValue);
}
}