View Compilation Error

0_0_29016784_22090.cpp: In function 'int main()':
0_0_29016784_22090.cpp:9:25: error: 'stdin' was not declared in this scope
     freopen("i.txt","r",stdin);
                         ^
0_0_29016784_22090.cpp:9:30: error: 'freopen' was not declared in this scope
     freopen("i.txt","r",stdin);
                              ^
0_0_29016784_22090.cpp:10:33: error: 'scanf' was not declared in this scope
     while(scanf("%lld%lld",&n,&m)==2)
                                 ^
0_0_29016784_22090.cpp:46:28: error: 'printf' was not declared in this scope
         printf("%lld\n",ans);
                            ^