/in/foo.c: In function 'main':
/in/foo.c:4:2: warning: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
scanf("%d%d",&a,&b);
^~~~~
/in/foo.c:4:2: warning: incompatible implicit declaration of built-in function 'scanf'
/in/foo.c:4:2: note: include '<stdio.h>' or provide a declaration of 'scanf'
/in/foo.c:5:2: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
printf("%d",(a+b)*(b-a+1)/2);
^~~~~~
/in/foo.c:5:2: warning: incompatible implicit declaration of built-in function 'printf'
/in/foo.c:5:2: note: include '<stdio.h>' or provide a declaration of 'printf'