function solution(common) {
var answer = 0;
var a1, a2, b1, b2;
a1 = common[1] - common[0];
a2 = common[2] - common[1];
b1 = common[1] / common[0];
b2 = common[2] / common[1];
answer = common[common.length - 1];
if (a1 === a2) {
answer = answer + a2;
}
if (b1 === b2) {
answer = answer * b2;
}
return answer;
}