Calculate：$$100-99+98-97+96-95+\cdots +4-3+2-1=$$． 
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$$=\left( 100-99 \right)+\left( 98-97 \right)+\left( 96-95 \right)+\cdots +\left( 4-3 \right)+\left( 2-1 \right)$$  $$=1+1+1+\cdots +1+1=50$$． 