已知等差数列{ana_nan}满足a1+a2+...+a14=77a_1+a_2+...+a_14=77a1+a2+...+a14=77,且a1a_1a1,a11∈Z+a_{11}∈Z_+a11∈Z+,则a18=a_{18}=a18= ______.