In a digital communication system, a symbol S randomly chosen from the set {s₁,s₂,s₃,s₄} is transmitted. It is given that s₁=-3, s₂=-1, s₃=+1 and s₄=+2. The received symbol is Y=S+W, where W is a zero-mean unit-variance Gaussian random variable and is independent of S. Pi is the conditional probability of symbol error for the maximum likelihood (ML) decoding when the transmitted symbol S=si. The index i for which the conditional symbol error probability Pi is the highest is