#include using namespace std; struct fast_ios { fast_ios(){ cin.tie(0); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_; #define FOR(i, begin, end) for(int i=(begin);i<(end);i++) #define REP(i, n) FOR(i,0,n) #define IFOR(i, begin, end) for(int i=(end)-1;i>=(begin);i--) #define IREP(i, n) IFOR(i,0,n) #define Sort(v) sort(v.begin(), v.end()) #define Reverse(v) reverse(v.begin(), v.end()) #define all(v) v.begin(),v.end() #define SZ(v) ((int)v.size()) #define Lower_bound(v, x) distance(v.begin(), lower_bound(v.begin(), v.end(), x)) #define Upper_bound(v, x) distance(v.begin(), upper_bound(v.begin(), v.end(), x)) #define Max(a, b) a = max(a, b) #define Min(a, b) a = min(a, b) #define bit(n) (1LL<<(n)) #define bit_exist(x, n) ((x >> n) & 1) #define Ans(f, y, n) if(f) cout << y << endl; else cout << n << endl; #define debug(x) cout << #x << "=" << x << endl; #define vdebug(v) cout << #v << "=" << endl; REP(i, v.size()){ cout << v[i] << ","; } cout << endl; #define mdebug(m) cout << #m << "=" << endl; REP(i, m.size()){ REP(j, m[i].size()){ cout << m[i][j] << ","; } cout << endl;} #define pb push_back #define f first #define s second #define int long long #define INF 1000000000000000000 using vec = vector; using mat = vector; using Pii = pair; using PiP = pair; using PPi = pair; using bools = vector; using pairs = vector; template void readv(vector &a){ REP(i, a.size()) cin >> a[i]; } void readv_m1(vector &a){ REP(i, a.size()){cin >> a[i]; a[i]--;} } //int dx[4] = {1,0,-1,0}; //int dy[4] = {0,1,0,-1}; int mod = 1000000007; //int mod = 998244353; #define Add(x, y) x = (x + (y)) % mod #define Mult(x, y) x = (x * (y)) % mod signed main(){ int N, L; cin >> N >> L; vec x(N); readv(x); vec y(N); readv(y); map m; for(int i: x) m[i] = 1; for(int i: y) m[i] = 2; int c = 0; int n = 0, d = 0; int tmp; for(auto p: m){ if(p.second == 2){ if(c == n) d = p.first; c = max(c - 1, 0LL); }else{ c++; Max(n, c); } } for(auto p: m){ if(p.second == 2){ if(c > n) d = p.first; c = max(c - 1, 0LL); if(c == 0) break; }else{ c++; Max(n, c); } } int ans = (n - 1) * L + d; cout << ans << endl; return 0; }