foo.c:1:1: warning: data definition has no type or storage class
    1 | a=input("请输入正方形边长，是一个整数：")
      | ^
foo.c:1:1: warning: type defaults to ‘int’ in declaration of ‘a’ [-Wimplicit-int]
foo.c:1:3: warning: implicit declaration of function ‘input’ [-Wimplicit-function-declaration]
    1 | a=input("请输入正方形边长，是一个整数：")
      |   ^~~~~
foo.c:1:3: error: initializer element is not constant
foo.c:2:1: error: expected ‘,’ or ‘;’ before ‘s’
    2 | s=a*a
      | ^