<?php

function equi ( $A ) {
    $count = count($A);
    $sum = array_sum($A) - $A[0];
    $i = 0;
    $sum_left = 0;
    while ($i < $count) {
        if ($sum_left == $sum)
            return $i;
        $sum_left += $A[$i];
        $sum -= $A[++$i];
    }
    return -1;
}