#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
using namespace std;
void *wmem;
char memarr[96000000];
template <class T>
inline void walloc1d(T **arr, int x, void **mem = &wmem) {
  static int skip[16] = {0, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1};
  (*mem) = (void *)(((char *)(*mem)) + skip[((unsigned long long)(*mem)) & 15]);
  (*arr) = (T *)(*mem);
  (*mem) = ((*arr) + x);
}
inline void rd(int &x) {
  int k;
  int m = 0;
  x = 0;
  for (;;) {
    k = getchar_unlocked();
    if (k == '-') {
      m = 1;
      break;
    }
    if ('0' <= k && k <= '9') {
      x = k - '0';
      break;
    }
  }
  for (;;) {
    k = getchar_unlocked();
    if (k < '0' || k > '9') {
      break;
    }
    x = x * 10 + k - '0';
  }
  if (m) {
    x = -x;
  }
}
inline void wt_L(char a) { putchar_unlocked(a); }
inline void wt_L(int x) {
  int s = 0;
  int m = 0;
  char f[10];
  if (x < 0) {
    m = 1;
    x = -x;
  }
  while (x) {
    f[s++] = x % 10;
    x /= 10;
  }
  if (!s) {
    f[s++] = 0;
  }
  if (m) {
    putchar_unlocked('-');
  }
  while (s--) {
    putchar_unlocked(f[s] + '0');
  }
}
inline void wt_L(const char c[]) {
  int i = 0;
  for (i = 0; c[i] != '\0'; i++) {
    putchar_unlocked(c[i]);
  }
}
template <class T>
struct LHeap {
  int *hp;
  int *place;
  int size;
  T *val;
  void malloc(int N) {
    hp = (int *)std::malloc(N * sizeof(int));
    place = (int *)std::malloc(N * sizeof(int));
    val = (T *)std::malloc(N * sizeof(T));
  }
  void walloc(int N, void **mem = &wmem) {
    walloc1d(&hp, N, mem);
    walloc1d(&place, N, mem);
    walloc1d(&val, N, mem);
  }
  void free() {
    std::free(hp);
    std::free(place);
    std::free(val);
  }
  void init(int N) {
    int i;
    size = 0;
    for (i = (0); i < (N); i++) {
      place[i] = -1;
    }
  }
  void up(int n) {
    int m;
    while (n) {
      m = (n - 1) / 2;
      if (val[hp[m]] <= val[hp[n]]) {
        break;
      }
      swap(hp[m], hp[n]);
      swap(place[hp[m]], place[hp[n]]);
      n = m;
    }
  }
  void down(int n) {
    int m;
    for (;;) {
      m = 2 * n + 1;
      if (m >= size) {
        break;
      }
      if (m + 1 < size && val[hp[m]] > val[hp[m + 1]]) {
        m++;
      }
      if (val[hp[m]] >= val[hp[n]]) {
        break;
      }
      swap(hp[m], hp[n]);
      swap(place[hp[m]], place[hp[n]]);
      n = m;
    }
  }
  void change(int n, T v) {
    T f = val[n];
    val[n] = v;
    if (place[n] == -1) {
      place[n] = size;
      hp[size++] = n;
      up(place[n]);
    } else {
      if (f < v) {
        down(place[n]);
      } else if (f > v) {
        up(place[n]);
      }
    }
  }
  int pop(void) {
    int res = hp[0];
    place[res] = -1;
    size--;
    if (size) {
      hp[0] = hp[size];
      place[hp[0]] = 0;
      down(0);
    }
    return res;
  }
};
struct graph {
  int N;
  int *es;
  int **edge;
  void setEdge(int N__, int M, int A[], int B[], void **mem = &wmem) {
    int i;
    N = N__;
    walloc1d(&es, N, mem);
    walloc1d(&edge, N, mem);
    for (i = (0); i < (N); i++) {
      es[i] = 0;
    }
    for (i = (0); i < (M); i++) {
      es[A[i]]++;
      es[B[i]]++;
    }
    for (i = (0); i < (N); i++) {
      walloc1d(&edge[i], es[i], mem);
    }
    for (i = (0); i < (N); i++) {
      es[i] = 0;
    }
    for (i = (0); i < (M); i++) {
      edge[A[i]][es[A[i]]++] = B[i];
      edge[B[i]][es[B[i]]++] = A[i];
    }
  }
};
template <class T>
struct wgraph {
  int N;
  int *es;
  int **edge;
  T **cost;
  graph g;
  void setEdge(int N__, int M, int A[], int B[], T C[], void **mem = &wmem) {
    int i;
    N = N__;
    walloc1d(&es, N, mem);
    for (i = (0); i < (N); i++) {
      es[i] = 0;
    }
    for (i = (0); i < (M); i++) {
      es[A[i]]++;
      es[B[i]]++;
    }
    walloc1d(&edge, N, mem);
    for (i = (0); i < (N); i++) {
      walloc1d(&edge[i], es[i], mem);
    }
    walloc1d(&cost, N, mem);
    for (i = (0); i < (N); i++) {
      walloc1d(&cost[i], es[i], mem);
    }
    for (i = (0); i < (N); i++) {
      es[i] = 0;
    }
    for (i = (0); i < (M); i++) {
      edge[A[i]][es[A[i]]] = B[i];
      edge[B[i]][es[B[i]]] = A[i];
      cost[A[i]][es[A[i]]++] = C[i];
      cost[B[i]][es[B[i]]++] = C[i];
    }
    g.N = N;
    g.es = es;
    g.edge = edge;
  }
};
int N;
int M;
int D[100000];
int A[200000];
int B[200000];
int ind[200000];
char S[100000 + 2];
int C[200000];
char rev[256];
int main() {
  wmem = memarr;
  int i;
  int j;
  int k;
  int x;
  wgraph<int> g;
  LHeap<int> hp;
  rev['W'] = 'B';
  rev['B'] = 'W';
  rd(N);
  rd(M);
  {
    int Lj4PdHRW;
    for (Lj4PdHRW = (0); Lj4PdHRW < (N); Lj4PdHRW++) {
      rd((D[Lj4PdHRW]));
    }
  }
  {
    int e98WHCEY;
    for (e98WHCEY = (0); e98WHCEY < (M); e98WHCEY++) {
      rd(A[e98WHCEY]);
      A[e98WHCEY] += (-1);
      rd(B[e98WHCEY]);
      B[e98WHCEY] += (-1);
    }
  }
  for (i = (0); i < (M); i++) {
    ind[i] = i;
  }
  g.setEdge(N, M, A, B, ind);
  hp.walloc(M);
  hp.init(M);
  for (i = (0); i < (M); i++) {
    hp.change(i, 1073709056);
  }
  for (i = (0); i < (M); i++) {
    if (D[A[i]] == D[B[i]]) {
      hp.change(i, D[A[i]]);
    }
  }
  while (hp.size) {
    int loop;
    k = hp.pop();
    if (hp.val[k] == 1073709056) {
      if (S[A[k]] == 0 || S[B[k]] == 0) {
        wt_L(-1);
        wt_L('\n');
        return 0;
      }
      C[k] = 1073709056;
      continue;
    }
    if (S[A[k]] && S[B[k]]) {
      C[k] = 1073709056;
      continue;
    }
    if (S[A[k]] == 0 && S[B[k]] == 0) {
      S[A[k]] = 'W';
    }
    if (S[A[k]] == 0) {
      S[A[k]] = rev[S[B[k]]];
    }
    if (S[B[k]] == 0) {
      S[B[k]] = rev[S[A[k]]];
    }
    C[k] = hp.val[k];
    for (loop = (0); loop < (2); loop++) {
      if (loop == 0) {
        i = A[k];
      } else {
        i = B[k];
      }
      for (j = (0); j < (g.es[i]); j++) {
        x = g.edge[i][j];
        if (S[x] != 0) {
          continue;
        }
        hp.change(g.cost[i][j], D[x]);
      }
    }
  }
  for (i = (0); i < (M); i++) {
    if (C[i] == 1073709056) {
      C[i] = 1000000000;
    }
  }
  wt_L(S);
  wt_L('\n');
  {
    int ZIeRIny5;
    for (ZIeRIny5 = (0); ZIeRIny5 < (M); ZIeRIny5++) {
      wt_L(C[ZIeRIny5]);
      wt_L('\n');
    }
  }
  return 0;
}
