foo.cc: In function 'double ojld(int, int)':
foo.cc:18:12: error: 'sqrt' was not declared in this scope
     return sqrt((p[i].x-p[j].x)*(p[i].x-p[j].x)
            ^~~~
foo.cc:18:12: note: suggested alternative: 'qsort'
     return sqrt((p[i].x-p[j].x)*(p[i].x-p[j].x)
            ^~~~
            qsort

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