View Compilation Error

0_0_33261212_17382.cpp: In function 'double dis(point, point)':
0_0_33261212_17382.cpp:9:32: error: 'sqrt' was not declared in this scope
     return sqrt(c.x*c.x+c.y*c.y);
                                ^
0_0_33261212_17382.cpp: In function 'int main()':
0_0_33261212_17382.cpp:29:4: error: 'cin' was not declared in this scope
    cin>>p[i].x>>p[i].y;     
    ^
0_0_33261212_17382.cpp:42:23: error: 'swap' was not declared in this scope
         swap(p[1],p[k]);//将最左下方的点即凸包上的第一个确定的点保存在p[1]中 
                       ^
0_0_33261212_17382.cpp:43:27: error: 'sort' was not declared in this scope
         sort(p+2,p+N+1,cmp);//按从小到大的顺序对各个点排序，距离p[1]比较近的p[2]也一定是凸包上的点 
                           ^