function solution(common) {
    var _a = [common[0], common[1], common[2]], a = _a[0], b = _a[1], c = _a[2];
    var len = common.length;
    if (a + c === 2 * b)
        return common[len - 1] + (b - a);
    else
        return common[len - 1] * (b / a);
}
var common = [1, 2, 3, 4];
console.log(solution(common));