#include <bits/stdc++.h>
using namespace std;
const int inf = 1e9;
inline void gmx(long long &x, long long y) {
  if (x < y) x = y;
}
inline void gmn(long long &x, long long y) {
  if (x > y) x = y;
}
inline int read() {
  char c = getchar();
  int z = 0, f = 1;
  while (c != '-' && (c > '9' || c < '0')) c = getchar();
  if (c == '-') f = -1, c = getchar();
  while (c >= '0' && c <= '9') z = (z << 1) + (z << 3) + c - '0', c = getchar();
  return z * f;
}
const int N = 1e4 + 5;
int n, c, p[N], s[N];
long long f[2][N], ans = 1e18;
int main() {
  n = read();
  c = read();
  for (int i = 1; i <= n; i++) p[i] = read();
  for (int i = 1; i <= n; i++) s[i] = read();
  for (int i = 1; i <= n; i++) {
    int o = i & 1;
    memset(f[o], 0x3f, sizeof(f[o]));
    f[o][0] = f[o ^ 1][0] + p[i];
    for (int j = 1; j <= i; j++)
      f[o][j] = min(f[o ^ 1][j] + 1ll * c * j + p[i], f[o ^ 1][j - 1] + s[i]);
  }
  for (int i = 0; i <= n; i++) gmn(ans, f[n & 1][i]);
  cout << ans;
  return 0;
}
