foo.cc: In function 'int main()':
foo.cc:72:15: warning: unused variable 'n3' [-Wunused-variable]
     int n1,n2,n3,i;
               ^~
foo.cc:72:18: warning: unused variable 'i' [-Wunused-variable]
     int n1,n2,n3,i;
                  ^

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