foo.c: In function 'main':
foo.c:4:2: warning: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
  scanf("%lld%lld%lld%lld", &a, &b, & c, &d);
  ^~~~~
foo.c:4:2: warning: incompatible implicit declaration of built-in function 'scanf'
foo.c:4:2: note: include '<stdio.h>' or provide a declaration of 'scanf'
foo.c:5:2: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
  printf("%lld", (a * d - b * c) % 1000000007);
  ^~~~~~
foo.c:5:2: warning: incompatible implicit declaration of built-in function 'printf'
foo.c:5:2: note: include '<stdio.h>' or provide a declaration of 'printf'

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