foo.c:2:1: error: unknown type name 'usingnamespace'
 usingnamespace std;
 ^~~~~~~~~~~~~~
foo.c: In function 'nmain':
foo.c:6:12: error: 'cin' undeclared (first use in this function)
     int n; cin >> n;
            ^~~
foo.c:6:12: note: each undeclared identifier is reported only once for each function it appears in
foo.c:8:13: warning: implicit declaration of function 'puts' [-Wimplicit-function-declaration]
             puts("even");
             ^~~~

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