View Compilation Error

0_0_33713006_5312.cpp: In member function 'void max_flow::work()':
0_0_33713006_5312.cpp:52:4: error: 'F' was not declared in this scope
    F.add_edge(0, i, 1);
    ^
0_0_33713006_5312.cpp:60:4: error: 'F' was not declared in this scope
    F.add_edge(l, r + len, inf);
    ^