find edges of vertices and sum of values at the edges' endpoints ?

[fenil]

Given N = 4, A = [1, 3], B = [2, 4] function should return 10. The graph contains two edges: (1, 2) and (3, 4). We can assign following values to the vertices: 4, 3, 2, 1.

 The sum of values at the edges' endpoints is (4 + 3) + (2 + 1) = 10.

 how to find edges of vertices and sum of values at the edges' endpoints ?

 

function ec($n, array $a, array $b){  }

[Barand]

Now it returns 10 again

function ec($n, array $a, array $b)
{
    return $n * $b[1] - $b[0] * $a[0] * $a[1] ;
}

echo ec( 4, [1,3], [2,4]);            // 10

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