What diseases did rats cause during WW1 ?
Find -LSB- math -RSB- S = \ sum _ -LCB- k = 1 -RCB- ^ n \ left -LRB- -1 \ right -RRB- ^ k \ begin -LCB- pmatrix -RCB- 2n-k \ \ k \ end -LCB- pmatrix -RCB- -LSB- / math -RSB- ?