foo.cpp: In function 'int main()':
foo.cpp:3:32: error: 'scanf' was not declared in this scope
     scanf("%d%d%d",&lenth,&n,&m);
                                ^
foo.cpp:26:20: error: 'printf' was not declared in this scope
     printf("%d",ans);
                    ^
foo.cpp:27:13: error: expected '}' at end of input
     return 0;
             ^