/in/foo.c: In function 'main':
/in/foo.c:8:8: error: redeclaration of 'k' with no linkage
    int k=0;int Sn=0;
        ^
/in/foo.c:6:20: note: previous declaration of 'k' was here
    int a, n, i, Sn,k;
                    ^
/in/foo.c:8:16: error: redeclaration of 'Sn' with no linkage
    int k=0;int Sn=0;
                ^~
/in/foo.c:6:17: note: previous declaration of 'Sn' was here
    int a, n, i, Sn,k;
                 ^~
/in/foo.c:9:18: warning: statement with no effect [-Wunused-value]
    for(i=0;i++;i <= n)
                ~~^~~~