1/1*2+1/2*3+1/3*4+...+1/199*200=1-1/2+1/2-1/3+1/3-1/4...+1/199-1/200=1-1/200=199/200
1-1/2+1/2-1/3+1/3-1/4+...+1/199-1/200=1-1/200=199/200
