Firstly note that C mod X1 is less that 10^6. So now create 10^6 nodes starting from 0, 1 ... to 10^6 — 1. Now for every node add an edge for every bucket i.e for node numbered i adding an edge of bucket j means adding edge between i and (i + bucket[j]) % X1. So adding edges like this we get X1 * (number of buckets) number of edges. Now basically our problem is reaching from 0 to C mod X1. So apply djikstra's algorithm with 0 as root and find the shortest route to C mod X1. If the value of shortest path is less than C then clearly we have a solution as coefficient of X1 could be increased appropriately, otherwise there is no solution as the shortest coefficient of X1 would have to be negative which is not allowed