View Compilation Error

0_0_28212109_31069.cpp: In function 'int main()':
0_0_28212109_31069.cpp:7:11: error: 'o' was not declared in this scope
 { int sum=o;
           ^
0_0_28212109_31069.cpp:12:8: error: 'o' was not declared in this scope
 return o;
        ^