Problem p01702 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, rm, lm;
  while (cin >> lm >> rm >> n, n + lm + rm) {
    int a[50][1002] = {};
    string
        pr =
            "000000000000000000000000000000000000000000000000000000000000000000"
            "000000000000000000000000000000000000000000000000000000000000000000"
            "000000000000000000000000000000000000000000000000000000000000000000"
            "000000000000000000000000000000000000000000000000000000000000000000"
            "000000000000000000000000000000000000000000000000000000000000000000"
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            "00000000000000000000000000000000",
        cl, cr;
    for (int o = 0; o < n; o++) {
      cin >> cl >> cr;
      for (int i = 0; i < lm; i++) {
        if (cl[i] == '1')
          for (int j = 0; j < rm; j++) {
            if (pr[j] == cr[j])
              a[i][j] = 1;
          }
        else
          for (int j = 0; j < rm; j++) {
            if (pr[j] != cr[j])
              a[i][j] = 1;
          }
      }
      pr = cr;
    }
    // for(int i=0;i<lm;i++)cout<<a[i][2];cout<<endl;
    for (int i = 0; i < rm; i++) {
      int p = -1, c = 0;
      for (int j = 0; j < lm; j++) {
        if (!a[j][i]) {
          if (c && p != j)
            c++;
          p = j;
          if (!c)
            c++;
        }
      }
      if (c == 1) {
        if (p < 10)
          cout << p;
        else
          cout << (char)('A' + p - 10);
      } else
        cout << '?';
    }
    cout << endl;
  }
}

Error: -11

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, rm, lm;
  while (cin >> lm >> rm >> n, n + lm + rm) {
    int a[100][100000] = {};
    string pl = "00000000000000000000000", pr = "0000000000000000000", cl, cr;
    for (int o = 0; o < n; o++) {
      cin >> cl >> cr;
      for (int i = 0; i < lm; i++) {
        if (cl[i] == '1')
          for (int j = 0; j < rm; j++) {
            if (pr[j] == cr[j])
              a[i][j] = 1;
          }
        else
          for (int j = 0; j < rm; j++) {
            if (pr[j] != cr[j])
              a[i][j] = 1;
          }
      }
      pr = cr;
    }
    for (int i = 0; i < rm; i++) {
      int p = -1, c = 0;
      for (int j = 0; j < lm; j++) {
        if (!a[j][i]) {
          if (c && p != j)
            c++;
          p = j;
          if (!c)
            c++;
        }
      }
      if (c == 1) {
        if (p < 10)
          cout << p;
        else
          cout << (char)('A' + p - 10);
      } else
        cout << '?';
    }
    cout << endl;
  }
}