=1/2000-1/2001+1/1999-1/2000+1/2001-1/1999=0=1/2+1/4+1/8+...+1/256+1/512+1/1024+1/1024-1/1024=1/2+1/4+1/8+...+1/256+1/512+1/512-1/1024=...=1-1/1024=1023/1024=(1+1999)*1999/2=2000/2*1999=1000*1999=1999...