IMO 1983 LL USS72

Prove that for all x1, x2, . . . , xn \inR the following inequality

IMO 1983 LL USS72

Origin: USS

Problem

Prove that for all x1, x2, . . . , xn \inR the following inequality holds:  n\geqi>j\geq1 cos2(xi −xj) \geqn(n −2) .