S 2+ 4+ 6+...+100 S 1 +3 +5......+99
S 100+ 98+ 96+...+ 2 S 99+ 97+95+... +1
-------------------------------------- ------------------------------
2S 102+ 102+ 102+... +102 2S 100+100+100+...+100
2S 102*50:2=2550 2S 100*50:2=2500
2500+X=2550 ; X=2500-2500 ; X=50