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<?php

$nums = array_map('intval', explode(' ', readline()));

$step = $nums[0];
$stepPrev = 0;
$sum = $step;

while(true){
    if (($stepPrev + $step) < count($nums)){
        $sum += $nums[$stepPrev+$step];
        $stepPrev += $step;
    }elseif (($stepPrev + $step) > count($nums) && ($stepPrev - $step) >= 0){
        $sum += $nums[$stepPrev-$step];
        $stepPrev -= $step;
    }else{
        echo $sum;
        return;
    }
    $step = $nums[$stepPrev];
}