No need to become a Reduce-Ninja, it's more important to understand the requirements (especially so for the exams).

;-)

function solve(input) {
    input.shift();
    let pairs = input.map(Number);

    let sums = pairs.reduce((acc, curr, i, arr) => {
        if (i === arr.length - 1) {
            return acc;
        } else if (i % 2 === 1) {
            return acc;
        }

        let currentSum = curr + arr[i + 1];
        acc.push(currentSum);

        return acc;

    }, [])

    let differences = pairs.reduce((acc, curr, i, arr) => {
        if (i === arr.length - 1) {
            return acc;
        } else if (i % 2 === 1) {
            return acc;
        }

        let currentDifference = curr + arr[i + 1];
        acc.push(currentDifference);

        return acc;

    }, [])

    let firstPair = sums[0];
    let allPairsEqual = sums.every((x) => x === firstPair);

    if (allPairsEqual) {
        console.log(`Yes, value=${firstPair}`);
    } else {
        let highestDifference = differences.reduce((acc, curr, i, arr) => {
            if (i === arr.length - 1) {
                return acc;
            }

            let currentDifference = Math.abs(curr - arr[i + 1]);

            if (currentDifference > acc) {
                acc = currentDifference;
            }

            return acc;
        }, 0);

        console.log(`No, maxdiff=${highestDifference}`);
    }
}