In a laboratory, a technician utilizes two different reagents, $R_1$ and $R_2$, to identify a compound. The probability of $R_1$ correctly identifying the compound is $p_1$, and $R_2$ is $p_2$. These reagents are used independently. If the probability that at least one reagent correctly identifies the compound is $0.91$ and the probability that exactly one correctly identifies it is $0.42$, calculate $p_1 + p_2$.