编程小白求助关于string.find_last_not_of()的问题

[detective336]

int main(){
    string a = "1,1900";
    string b = "0123456789";
    string::size_type pos0 = a.find_first_not_of(b, 0);
    string::size_type pos1 = a.find_last_not_of(b, 0);
    cout<<"pos0: "<<pos0<<"\npos1: "<<pos1<<endl;
    return 0;
}

程序的输出结果为什么是这样的:
pos0: 1
pos1: 18446744073709551615

pos0为什么不等于pos1?????
求解答～谢谢