But when n + 1 {\displaystyle n+1} is not prime, the first factor becomes zero and the formula produces the prime number 2.[1] This formula is not an efficient way to generate prime numbers because evaluating n ! mod ( n + 1 ) {\displaystyle n!{\bmod {(}}n+1)} requires about n − 1 {\displaystyle n-1} multiplications and reductions mod ( n + 1 ) {\displaystyle {\bmod {(}}n+1)} .