View Compilation Error

0_0_28260809_21408.cpp: In function 'int main()':
0_0_28260809_21408.cpp:43:38: error: invalid initialization of non-const reference of type 'int&' from an rvalue of type 'int'
   printf("%d\n",add(dfs(len-1,0,0,1,1),P-1));
                                      ^
0_0_28260809_21408.cpp:17:13: note: in passing argument 1 of 'void add(int&, int)'
 inline void add(int&a,int b){a+=b;if(a>=P)a-=P;}
             ^