C++ :

#include <cstdio>
#include <algorithm>
using namespace std;
struct Shop { int p, c; };
static Shop s[1005];
bool cmp(const Shop &x, const Shop &y){ return x.p < y.p; }
/* 每行是「单价 存货量」，最小花费 ⇒ 按单价升序买，买光一家再下一家。 */
int main(){
    int m, n;
    if(scanf("%d %d", &m, &n) != 2) return 0;
    for(int i = 0; i < n; i++) scanf("%d %d", &s[i].p, &s[i].c);
    sort(s, s + n, cmp);
    long long need = m, cost = 0;
    for(int i = 0; i < n && need > 0; i++){
        long long take = s[i].c < need ? s[i].c : need;
        cost += take * s[i].p;
        need -= take;
    }
    printf("%lld\n", cost);
    return 0;
}