View Compilation Error

0_0_14136872_24287.cpp: In function 'void solve()':
0_0_14136872_24287.cpp:138:26: error: 'mod' was not declared in this scope
   x2[i] = a[sum - i].y % mod ;
                          ^
0_0_14136872_24287.cpp:149:20: error: 'mod' was not declared in this scope
   x1[i] = a[i].y % mod ;
                    ^