#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ar array
void solve() {
int n,p;
cin >> n >> p;
vector<int> a(n),b(n),k(p + 1,0),check(p,0);
for (int i = 1; i <= p;i++) {
cin >> k[i];
k[i] +=k[i - 1];
}
for (int i = 0; i < n;i++) cin >> a[i] >> b[i];
ll ans = 0;
for (int i = 30; i >= 0;i--) {
bool ok = 1;
int set = 1 << i;
for (int j = 0; j < p;j++) {
if (check[j]) continue;
bool kk = 0;
for (int v = k[j]; v < k[j + 1];v++) {
kk |= (b[v] >= set);
}
ok &= kk;
}
for (int j = 0; j < p;j++) {
int q = 0, q1 = 0;
for (int v = k[j];v < k[j + 1];v++) {
q += (b[v] >= set);
q1 += (b[v] >= set && a[v] < set);
}
for (int v = k[j];v < k[j + 1];v++) {
if (q1 > 0 && (!ok || q > 1)) check[j] = 1;
if (b[v] < set) continue;
a[v] -= set;
b[v] -= set;
}
}
if (ok) ans += set;
}
cout << ans;
}
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
int t = 1;
// cin >> t;
while (t--) {
solve();
cout << "\n";
}
return 0;
}
