The method is as follows: one board is placed on the corner of the square moat, which is equivalent to an isosceles right triangle. The hypotenuse =4, so the height on the hypotenuse =2√2, and the other board is placed on the other side of the river. A 4m-wide moat with an angle width of 4√2≈5.65m, and the span width of the above two slabs is 4+2 √ 2 = 4+2.828 = 6.83 > 5.65. So you can cross the river.