Making Exceptions in a Grid

let rows = 5;
let columns = 6;

function setup() {
  createCanvas(windowWidth, windowHeight);
}

function draw() {
  background(200);
  for (let row = 0; row < rows; row++) {
    for (let column = 0; column < columns; column++) {
      let x = 30 + 50 * column;
      let y = 40 + 50 * row;

      square(x, y, 30);
    }
  }
}