/in/foo.c: In function 'main':
/in/foo.c:7:9: error: 'sum1' undeclared (first use in this function); did you mean 'sum'?
    7 |         sum1=a+b;
      |         ^~~~
      |         sum
/in/foo.c:7:9: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:8:9: error: 'sum2' undeclared (first use in this function); did you mean 'sum'?
    8 |         sum2=b+c;
      |         ^~~~
      |         sum
/in/foo.c:9:9: error: 'sum3' undeclared (first use in this function); did you mean 'sum'?
    9 |         sum3=a+c;
      |         ^~~~
      |         sum