Conditional probability in bivariate normal

Let $(X, Y)$ be a random vector having bivariate normal distribution with parameters $E(X)=0, \mathrm{Var}(X)=1, E(Y)=-1, \mathrm{Var}(Y)=4$ and $\rho(X, Y)=-\frac{1}{2}$, where $\rho(X, Y)$ denotes the correlation coefficient between $X$ and $Y$. Then $P(X+Y > 1 \mid 2X-Y=1)$ equals