How many numbers must be deleted from the multiplication formula 1*2*3*4*...*26*27 so that the product of the remaining numbers is a perfect square number?
从乘法算式1*2*3*4*…*26*27中最少要删掉多少个数，才能使得剩下的数的乘积是个完全平方数？
<result>5</result>