#include <stdio.h>

int main() {
    int n, m;
    scanf("%d %d", &n, &m);
    
    int package = 1000, each = 1000, p, e;
    for (int i = 0; i < m; i++) {
        scanf("%d %d", &p, &e);
        package = p < package ? p : package;
        each = e < each ? e : each;
    }
    
    int money;
	
    if (package > each * 6) {
        money = n * each;
    } else if (n % 6 * each > package) {
        money = ((n / 6)+ 1) * package;
    } else {
        money = (n % 6) * each + (n / 6) * package;
    }
    
    printf("%d", money);
    
    return 0;
}

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MTAgMwoyMCA4CjQwIDcKNjAgNA==

10 3
20 8
40 7
60 4