#include <stdio.h>
int find(float a[], int n) {
    int max = a[0], r = 0;
    for (int i = 1; i < n; i++) {
        if (a[i] > max) {
            max = a[i];
            r = i;
        }
    }
    return r;
}
int main() {
    int find(float a[], int n);
    int v, n;
    scanf("%d %d", &v, &n);
    float a[n], b[n];
    for (int i = 0; i < n; i++) {
        scanf("%f %f", &a[i], &b[i]);
    }
    float value = 0;
    while (1) {
        int j = find(b, n);
        float temp = b[j];
        b[j] = 0;
        if (v - a[j] > 0) {
            v = v - a[j];
            value = value + temp * a[j];
            continue;
        } else {
            value = value + v * temp;
            break;
        }
    }
    printf("%g", value);
    return 0;
}