#include <fstream>

using namespace std;

ifstream in("energii.in");
ofstream out("energii.out");

const int N=10005, MAX=1000000000;
int n, k, maxi, mini=MAX;
int profit[N], p[N], g[N];

void citire()
{
    in>>n>>k;
    for(int i=1; i<=k; i++)
    {
        in>>g[i]>>p[i];
        maxi=max(maxi, g[i]);
    }
}

int main()
{
    citire();
    for(int i=1; i<=k+maxi; i++) profit[i]=MAX;
    for(int i=1; i<=n; i++)
        for(int j=k-1; j>=0; j--)
            if(profit[j]!=MAX && profit[j]+p[i]<profit[j+g[i]]) profit[j+g[i]]=profit[j]+p[i];
    for(int i=k; i<=k+maxi; i++) mini=min(mini, profit[i]);
    if(mini==MAX) out<<-1;
    else out<<mini;
    return 0;
}