foo.cc: In function 'bool spfa()':
foo.cc:19:9: warning: unused variable 'num' [-Wunused-variable]
     int num = 0;
         ^~~
foo.cc: In function 'int main()':
foo.cc:45:10: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d%d", &n, &m);
     ~~~~~^~~~~~~~~~~~~~~~
foo.cc:49:14: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
         scanf("%d%d%d", &x, &a, &b);
         ~~~~~^~~~~~~~~~~~~~~~~~~~~~

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