Main.cc: In function 'int main()': Main.cc:5:27: error: 'abs' was not declared in this scope if((a+b>c)&&(abs(a-b>c))){ ^ Main.cc:4:29: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result] scanf("%d%d%d",&a,&b,&c); ^