merging and squashing with Number

const input = ['4', '12', '23', '34', '45'];
const gets = this.gets || ((arr, index) => () => arr[index++])(input, 0);
const print = this.print || console.log;

let n = +gets();
let row1 = [];
let row2 = [];

let arr = [];

for (let i = 1; i <= n; i++) {
    let num = +gets();
    arr.push(num)
}

for (let i = 0; i < arr.length - 1; i++) {
    let firstNumber = arr[i] % 10;
    let secondNumber = Math.floor(arr[i + 1] / 10)

    let firNum = Math.floor(arr[i] / 10);
    let secNum = Math.floor(arr[i + 1] % 10);
    let middle = firstNumber + secondNumber;
    if (middle > 9) {
        middle = middle % 10;
    }
    row1.push(firstNumber + "" + secondNumber);
    row2.push(firNum + "" + middle + "" + secNum)
}

print(row1.join(' '));
print(row2.join(' '));