#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <algorithm>#define IOS ios::sync_with_stdio(false)
using namespace std;
#define inf (0x3f3f3f3f)
typedef long long int LL;#include <iostream>
#include <sstream>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <string>
#include <bitset>
const int maxn = 1e3 + 20;
int t[maxn];
int val[maxn];
int T, H, n;
int mx[maxn];
int dp_max[maxn][20];
void init() {for (int i = 1; i <= n; ++i) {int tot = 0, useTime = 0;for (int j = i; j <= n; ++j) {useTime += t[j];if (useTime > T) break;tot += val[j];}mx[i] = tot;}for (int i = 1; i <= n; ++i) {dp_max[i][0] = mx[i];}for (int j = 1; j < 20; ++j) {for (int i = 1; i + (1 << j) - 1 <= n; ++i) {dp_max[i][j] = max(dp_max[i][j - 1], dp_max[i + (1 << (j - 1))][j - 1]);}}
}
int ask(int be, int en) {if (be >  en) return 0;int k = (int)log2(en - be + 1.0);return max(dp_max[be][k], dp_max[en - (1 << k) + 1][k]);
}
void work() {scanf("%d%d%d", &T, &H, &n);int hurt = 0;for (int i = 1; i <= n; ++i) {scanf("%d%d", &t[i], &val[i]);hurt += val[i];}init();int mxsave = 0, ans = 0;for (int i = 1; i <= n; ++i) {int useTime = 0;int tot = 0;for (int j = i; j <= n; ++j) {useTime += t[j];if (useTime > 2 * T) {break;}tot += val[j];}mxsave = max(mxsave, tot);}ans = H - (hurt - mxsave);for (int i = 1; i <= n; ++i) {int tot = 0, useTime = 0;for (int j = i; j <= n; ++j) {useTime += t[j];if (useTime > T) break;tot += val[j];int res = ask(j + 1, n);mxsave = max(mxsave, tot + res);}}ans = max(ans, H - (hurt - mxsave));ans = max(ans, 0);cout << ans << endl;
}int main() {
#ifdef localfreopen("data.txt", "r", stdin);
//    freopen("data.txt", "w", stdout);
#endifwork();return 0;
}