function solution(common: number[]) {
    let [a, b, c]: number[] = [common[0], common[1], common[2]];
    let len: number = common.length;

    if(a + c === 2 * b) return common[len - 1] + (b - a);
    else return common[len - 1] * (b / a);
}

let common: number[] = [1,2,3,4];
console.log(solution(common));