add and substract

function  solve(arr){

    let originalSum = arr.reduce((a, b) => a + b);

    for (let i = 0; i < arr.length; i++) {
        if (arr[i] % 2 === 0){
            let number = arr[i];
            arr[i] = number + i;
        }
        else {
            let number = arr[i];
            arr[i] = number - i;
        }
    }

    let newSum = arr.reduce((a, b) => a + b);

    console.log(arr);
    console.log(originalSum);
    console.log(newSum);
}