多元正态分布在R中的数值积分

# x1 and x2 are bivariate normal distributed with 
mu1=mu2=0
cov=matrix(c(1,1/2,1/2,1),2,2)

# trying to find the solution satisfying the equality with  t1=t2
# p(x1>t1) + p(x1<t2,x2>t2) = 0.05