Assume that a，b，c，d are integers，and four equations （a-2b）x=1，（b-3c）y=1，（c-4d）z=1，w+100=d have always solutions x，y，z，w of positive numbers respectively，then the minimum of a is
______． （英汉词典：to assume假设；integer整数；equation方程；solutions （方程的）解；positive正的；respectively分别地；minimum最小值）!
