Binomial Theorem

For the natural numbers m, n, if \({(1 - y)^m}{(1 + y)^n} = 1 + {a_1}y + {a_2}{y^2} + .... + {a_{m + n}}{y^{m + n}}\) and \({a_1} = {a_2} = 10\), then the value of (m + n) is equal to :