foo.c:2:1: error: unknown type name 'using'
    2 | using namespace std;
      | ^~~~~
foo.c:2:17: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
    2 | using namespace std;
      |                 ^~~
foo.c: In function 'main':
foo.c:4:5: error: 'x1' undeclared (first use in this function)
    4 | {   x1=int(input("x1:"))
      |     ^~
foo.c:4:5: note: each undeclared identifier is reported only once for each function it appears in
foo.c:4:8: error: expected expression before 'int'
    4 | {   x1=int(input("x1:"))
      |        ^~~

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