(2-1)+(3-2)+(4-3+...+(20-19)=
=(2+3+4+...+19+20)-(1+2+3+..+18+19)
=(2+3+4+...+19)+20 -1 -(2+3+..+18+19)
=20-1
=19
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(2-1)+(3-2)+(4-3+...+(20-19)=
=(2+3+4+...+19+20)-(1+2+3+..+18+19)
=(2+3+4+...+19)+20 -1 -(2+3+..+18+19)
=20-1
=19