View Compilation Error

0_0_13899933_23030.cpp:3:12: error: '::main' must return 'int'
 void main ()
            ^
0_0_13899933_23030.cpp: In function 'int main()':
0_0_13899933_23030.cpp:10:21: error: 'strlen' was not declared in this scope
   for(i=0;i<strlen(a);i=i+2)
                     ^