#include <cstdio>

#include <cstring>

#include <iostream>

#include <algorithm>

using namespace std;

typedef long long ll;

const int inf = 0x7fffffff;

int n, m, u[], v[];

ll ans = inf;

void erfen(int cur)

{

    int l = , r = ,res = ;

    while (l <= r)

    {

        int mid = (l + r) >> ;

        if (1LL * u[cur] * (n - mid) - 1LL * v[cur] * mid >= )

        {

            res = mid;

            l = mid + ;

        }

        else

            r = mid - ;

    }

    ans = min(ans, 1LL * u[cur] * (n - res) - 1LL * v[cur] * res);

}

int main()

{

    scanf("%d%d", &n, &m);

    for (int i = ; i <= m; i++)

        scanf("%d%d", &u[i], &v[i]);

    for (int i = ; i <= m; i++)

        erfen(i);

    printf("%lld\n", ans);

    return ;

}