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In a digital communication system, a symbol $S$ randomly chosen from the set $\left \{ s_{1},s_{2},s_{3},s_{4} \right \}$ is transmitted. It is given that $s_{1}=-3,s_{2}=-1,s_{3}=+1$ and $s_{4}=+2$. The received symbol is $Y=S+W$ . $W$ is a zero-mean unit-variance Gaussian random variable and is independent of $S$. $P_{i}$ is the conditional probability of symbol error for the maximum likelihood $\text{(ML)}$ decoding when the transmitted symbol $S=s_{i}$. The index $i$ for which the conditional symbol error probability $P_{i}$ is the highest is ___________.