View Compilation Error

0_0_17235375_11635.cpp: In function 'double cal(int, double, double)':
0_0_17235375_11635.cpp:11:41: error: 'sqrt' was not declared in this scope
  double res = sqrt(a*a + (y - b)*(y - b));
                                         ^
0_0_17235375_11635.cpp: In function 'int main()':
0_0_17235375_11635.cpp:29:20: error: 'sqrt' was not declared in this scope
   s[i + 1] = sqrt(2) * i + sqrt(i * i + (i + 1)*(i + 1)) + s[i];
                    ^