#include <bits/stdc++.h>
using namespace std;
int n, i, j, k, a[1000005], b[1000005], c[2], cnt, p[1000005];
int t[1000005 * 4][2];
void maketree(int x, int l, int r) {
  if (l == r) {
    t[x][0] = t[x][1] = 0, t[x][p[l]] = l;
    return;
  }
  int mid = (l + r) >> 1;
  maketree(x << 1, l, mid), maketree(x << 1 ^ 1, mid + 1, r);
  t[x][0] = (t[x << 1][0]) ? t[x << 1][0] : t[x << 1 ^ 1][0];
  t[x][1] = (t[x << 1][1]) ? t[x << 1][1] : t[x << 1 ^ 1][1];
}
void change(int x, int l, int r, int v) {
  if (l == r) {
    t[x][0] = t[x][1] = 0;
    return;
  }
  int mid = (l + r) >> 1;
  if (v <= mid)
    change(x << 1, l, mid, v);
  else
    change(x << 1 ^ 1, mid + 1, r, v);
  t[x][0] = (t[x << 1][0]) ? t[x << 1][0] : t[x << 1 ^ 1][0];
  t[x][1] = (t[x << 1][1]) ? t[x << 1][1] : t[x << 1 ^ 1][1];
}
int find(int x, int l, int r, int L, int R, int v) {
  if (l > R || r < L) return 0;
  if (L <= l && r <= R) return t[x][v];
  int mid = (l + r) >> 1, tmp = find(x << 1, l, mid, L, R, v);
  if (tmp)
    return tmp;
  else
    return find(x << 1 ^ 1, mid + 1, r, L, R, v);
}
int main() {
  scanf("%d", &n);
  char ch = getchar();
  while (ch < '0' || ch > '1') ch = getchar();
  for (i = 1; i <= n; i++) a[i] = ch - '0', ch = getchar();
  while (ch < '0' || ch > '9') ch = getchar();
  for (i = 1; i <= n; i++) b[i] = ch - '0', ch = getchar();
  for (i = 1; i <= n; i++)
    if (a[i] != b[i]) p[++cnt] = a[i] > b[i];
  int res = 0, ans = 0;
  for (i = 1; i <= cnt; i++) res += p[i] ? 1 : -1;
  if (res != 0) printf("-1"), exit(0);
  if (!cnt) printf("0"), exit(0);
  maketree(1, 1, cnt);
  res = cnt;
  while (res) {
    ans++;
    int x = find(1, 1, cnt, 1, cnt, 0), y = find(1, 1, cnt, 1, cnt, 1), v,
        now = max(x, y);
    change(1, 1, cnt, x), change(1, 1, cnt, y), res -= 2;
    if (x > y)
      v = 1;
    else
      v = 0;
    if (!res) break;
    x = find(1, 1, cnt, now + 1, cnt, v);
    while (x) {
      y = find(1, 1, cnt, x + 1, cnt, v ^ 1);
      if (y) {
        change(1, 1, cnt, x), change(1, 1, cnt, y), res -= 2;
        now = y;
        if (!res) break;
        x = find(1, 1, cnt, now + 1, cnt, v);
      } else
        break;
    }
  }
  printf("%d\n", ans);
}
