#/***********************************************************
#    area.rb -- ллюя
#***********************************************************/

def area( n,  x,  y)
    a = x[n - 1] * y[0] - x[0] * y[n - 1]
    for i in 1...n
        a += x[i - 1] * y[i] - x[i] * y[i - 1]
    end
    return 0.5 * a
end

x = [ 1, 3, 2, 0 ]; y = [ 1, 2, 4, 2 ]
printf("ллюя = %g\n", area(4, x, y))
exit 0