View Compilation Error

0_0_22200936_6029.cpp:2:12: error: '::main' must return 'int'
 float main()
            ^
0_0_22200936_6029.cpp: In function 'int main()':
0_0_22200936_6029.cpp:7:14: error: 'scanf' was not declared in this scope
 scanf("%g",&r);
              ^
0_0_22200936_6029.cpp:9:17: error: 'printf' was not declared in this scope
 printf("%3g\n",v);
                 ^