View Compilation Error

0_0_29975398_27168.cpp: In function 'void dfs(int, int)':
0_0_29975398_27168.cpp:20:7: error: invalid operands of types 'int' and 'void' to binary 'operator+'
   sum += dfs(v, u);
       ^
0_0_29975398_27168.cpp:20:7: error:   in evaluation of 'operator+=(int, void)'