#include <bits/stdc++.h>
#define scanf scanf_s
using namespace std;
int a, n, v, b[200005], maxvalue, maxv, i;

int main() {
    bool cmp(int a, int b);
    scanf("%d%d", &v, &n);
    map<int, int> juice;
    for (i = 0; i < n; i++) {
        scanf("%d%d", &a, b + i);
        if (juice[b[i]])
            juice[b[i]] += a;
        else
            juice[b[i]] = a;
    }
    sort(b, b + n, cmp);
    for (i = 0; i < n && maxv < v; i++) {
        maxvalue += b[i] * juice[b[i]];
        maxv += juice[b[i]];
    }
    if (maxv > v)
        maxvalue -= (maxv - v) * b[i - 1];
    printf("%d", maxvalue);
    return 0;
}

bool cmp(int a, int b) { return a > b; }

/sandbox/1/a.cpp: In function 'int main()':
/sandbox/1/a.cpp:2:15: error: 'scanf_s' was not declared in this scope
 #define scanf scanf_s
               ^~~~~~~
/sandbox/1/a.cpp:9:2: note: in expansion of macro 'scanf'
  scanf("%d%d", &v, &n);
  ^~~~~
/sandbox/1/a.cpp:2:15: note: suggested alternative: 'scanf'
 #define scanf scanf_s
               ^~~~~~~
/sandbox/1/a.cpp:9:2: note: in expansion of macro 'scanf'
  scanf("%d%d", &v, &n);
  ^~~~~