View Compilation Error

0_0_39832600_1411.cpp: In function 'int main()':
0_0_39832600_1411.cpp:44:22: error: 'f' was not declared in this scope
   44 |             int s2 = f[x2][y2] - f[x1 - 1][y2] - f[x2][y1 - 1] + f[x1 - 1][y1 - 1];
      |                      ^