#include <map>
#include <set>
#include <list>
#include <queue>
#include <stack>
#include <vector>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <algorithm>

using namespace std;

int dp[110][110];
const int inf = 0x3f3f3f3f;

struct node
{
	int w;
	int c;
}guaiwu[110];

int main()
{
	int n, m, k, s;
	while (~scanf("%d%d%d%d", &n, &m, &k, &s))
	{
		memset (dp, 0, sizeof(dp));
		int ans = inf;
		for (int i = 1; i <= k; ++i)
		{
			scanf("%d%d", &guaiwu[i].w, &guaiwu[i].c);
		}
		for (int i = 1; i <= k; ++i)
		{
			for (int j = 1; j <= s; ++j)
			{
				for (int l = guaiwu[i].c; l <= m; ++l)
				{
					dp[j][l] = max(dp[j][l], dp[j - 1][l - guaiwu[i].c] + guaiwu[i].w);
					if (dp[j][l] >= n)
					{
						ans = min(ans, l);
					}
				}
			}
		}
		if (dp[s][m] < n)
		{
			printf("-1\n");
			continue;
		}
		printf("%d\n", m - ans);
	}
	return 0;
}