foo.c:1:1: warning: return type defaults to 'int' [-Wimplicit-int]
 main(a,b){scanf("%d%d",&a,&b);printf("%d",a+b);}
 ^~~~
foo.c: In function 'main':
foo.c:1:1: warning: type of 'a' defaults to 'int' [-Wimplicit-int]
foo.c:1:1: warning: type of 'b' defaults to 'int' [-Wimplicit-int]
foo.c:1:11: warning: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
 main(a,b){scanf("%d%d",&a,&b);printf("%d",a+b);}
           ^~~~~
foo.c:1:11: warning: incompatible implicit declaration of built-in function 'scanf'
foo.c:1:11: note: include '<stdio.h>' or provide a declaration of 'scanf'
foo.c:1:31: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
 main(a,b){scanf("%d%d",&a,&b);printf("%d",a+b);}
                               ^~~~~~
foo.c:1:31: warning: incompatible implicit declaration of built-in function 'printf'
foo.c:1:31: note: include '<stdio.h>' or provide a declaration of 'printf'

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