foo.c: In function 'main':
foo.c:8:14: warning: implicit declaration of function 'sum' [-Wimplicit-function-declaration]
  printf("%d",sum(c(n,m)));
              ^~~
foo.c:8:18: warning: implicit declaration of function 'c' [-Wimplicit-function-declaration]
  printf("%d",sum(c(n,m)));
                  ^
foo.c:6:5: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d %d",&m,&n);
     ^~~~~~~~~~~~~~~~~~~~

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