#include<stdio.h>

int main () {

int n;

scanf("%d",&n);

int s[n][5];

for(int i = 0; i < n; i++) {

for(int j = 0; j < 4; j++) {

scanf("%d",&s[i][j]);

}

}

for(int i = 0; i < n; i++) {

if(s[i][3] - s[i][2] == s[i][2] - s[i][1] && s[i][2] - s[i][1] == s[i][1] - s[i][0] ) {

s[i][4] = s[i][3] + s[i][2] - s[i][1];

} else {

s[i][4] = s[i][3] * s[i][2] / s[i][1];

}