Problem:
Score : 500 points

Problem Statement

Ibis is fighting with a monster.

The health of the monster is H.

Ibis can cast N kinds of spells. Casting the i-th spell decreases the monster's health by A_i, at the cost of B_i Magic Points.

The same spell can be cast multiple times. There is no way other than spells to decrease the monster's health.

Ibis wins when the health of the monster becomes 0 or below.

Find the minimum total Magic Points that have to be consumed before winning.

Constraints

1 \leq H \leq 10^4

1 \leq N \leq 10^3

1 \leq A_i \leq 10^4

1 \leq B_i \leq 10^4

All values in input are integers.

Input

Input is given from Standard Input in the following format:

H N
A_1 B_1
:
A_N B_N

Output

Print the minimum total Magic Points that have to be consumed before winning.

Sample Input 1

9 3
8 3
4 2
2 1

Sample Output 1

4

First, let us cast the first spell to decrease the monster's health by 8, at the cost of 3 Magic Points. The monster's health is now 1.

Then, cast the third spell to decrease the monster's health by 2, at the cost of 1 Magic Point. The monster's health is now -1.

In this way, we can win at the total cost of 4 Magic Points.

Sample Input 2

100 6
1 1
2 3
3 9
4 27
5 81
6 243

Sample Output 2

100

It is optimal to cast the first spell 100 times.

Sample Input 3

9999 10
540 7550
691 9680
700 9790
510 7150
415 5818
551 7712
587 8227
619 8671
588 8228
176 2461

Sample Output 3

139815

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(ll i = 0; i < (n); ++i)
#define repA(i, a, n) for(int  i = a; i <= (n); ++i)
#define all(x) x.begin(), x.end()
#define sz(x) (int) (x).size()
#define fill(a) memset(a, 0, sizeof(a))
#define fst first
#define snd second
#define mp make_pair
#define pb push_back
#define endl '\n'
#define init(n) int n;cin>>n;int a[n];rep(i,n)cin>>a[i];
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<double> vd;
typedef vector<long long int> vll;
typedef vector<pii> vii;
ll p=1000000007;
ll const N=1e6+5;
struct node{
	ll x,h;
};
bool comp(node a,node b){
	return a.x<b.x;
}
ll getSum(ll BITree[], ll index) 
{ 
    ll sum = 0; // Iniialize result 
  
    // index in BITree[] is 1 more than the index in arr[] 
    index = index + 1; 
  
    // Traverse ancestors of BITree[index] 
    while (index>0) 
    { 
        // Add current element of BITree to sum 
        sum += BITree[index]; 
  
        // Move index to parent node in getSum View 
        index -= index & (-index); 
    } 
    return sum; 
} 
void updateBIT(ll BITree[], ll n, ll index, ll val) 
{ 
    index = index + 1; 
  
    // Traverse all ancestors and add 'val' 
    while (index <= n) 
    { 
    // Add 'val' to current node of BI Tree 
    BITree[index] += val; 
  
    // Update index to that of parent in update View 
    index += index & (-index); 
    } 
} 
int main() {
    
    cin.sync_with_stdio(0); cin.tie(0);
    cin.exceptions(cin.failbit);
    int T;T=1;
    // cin>>T;
    while(T--){
   		ll ans=0;
   		int h,n;cin>>h>>n;
   		int a[n],b[n];
   		rep(i,n){
   			cin>>a[i]>>b[i];
   		}
   		int dp[h+1]={0};
   		rep(i,h+1)dp[i]=INT_MAX;
   		dp[0]=0;
   		repA(i,1,h){
   			for(int j=0;j<n;j++){
   				int t=max(0,i-a[j]);
   				dp[i]=min(b[j]+dp[t],dp[i]);
   			}
   		}
   		cout<<dp[h]<<endl;
    }
    return 0;   
}