View Compilation Error

0_0_14800102_24419.cpp: In function 'void dfs(node)':
0_0_14800102_24419.cpp:37:76: error: 'abs' was not declared in this scope
    double num = abs((map[next.x][next.y] - '0') - (map[now.x][now.y] - '0'));
                                                                            ^