From the first \( 100 \) natural numbers, two numbers first \( a \) and then \( b \) are selected randomly without replacement. If the probability that \( a - b \ge 10 \) is \( \frac{m}{n} \), \( \gcd(m, n) = 1 \), then \( m + n \) is equal to \( \_\_\_\_\_ \).
