View Compilation Error

0_0_30128023_2720.cpp: In function 'int main()':
0_0_30128023_2720.cpp:10:34: error: 'mod' was not declared in this scope
         a[i] = (a[i-1] + a[i-3])%mod;
                                  ^
0_0_30128023_2720.cpp:27:36: error: 'mod' was not declared in this scope
             printf("%lld\n",a[len]%mod);
                                    ^