Main.c:2:7: warning: type defaults to 'int' in declaration of 'double\U000003a0' [-Wimplicit-int]
    2 | const doubleΠ=3.1415926;
      |       ^~~~~~~
Main.c: In function 'main':
Main.c:6:13: warning: format '%f' expects argument of type 'float *', but argument 2 has type 'double *' [-Wformat=]
    6 |     scanf("%f",&n);
      |            ~^  ~~
      |             |  |
      |             |  double *
      |             float *
      |            %lf
Main.c:8:9: error: '\U000003a0' undeclared (first use in this function)
    8 |     b=4*Π*n*n;
      |         ^
Main.c:8:9: note: each undeclared identifier is reported only once for each function it appears in
Main.c:9:18: warning: format '%f' expects a matching 'double' argument [-Wformat=]
    9 |     printf("%f%f%f",a,b);
      |                 ~^
      |                  |
      |                  double