View Compilation Error

0_0_25514573_24992.cpp: In function 'void solve()':
0_0_25514573_24992.cpp:60:40: error: 'fab' was not declared in this scope
                 ans += 1LL*(fab(x1 - x2) + 1) * (fab(y1 - y2) + 1);
                                        ^