p(A) (U(decrypt in 1 month) - cost(1 month computer time)) + (1 - p(A)) (U(decrypt in 2 months) - cost(2 months computer time))
p(A) (U(decrypt in 1 month) - cost(1 month computer time)) + (1 - p(A)) (U(decrypt in 2 months) - cost(2 months computer time))