$$=\left( 100-99 \right)+\left( 98-97 \right)+\left( 96-95 \right)+\cdots +\left( 4-3 \right)+\left( 2-1 \right)$$  $$=1+1+1+\cdots +1+1=50$$． 
Calculate：$$100-99+98-97+96-95+\cdots +4-3+2-1=$$． 
1: $$40$$ 
2: $$45$$ 
3: $$49$$ 
4: $$50$$ 