include <stdio.h>

int main() {

int n, m, x, y, a, M = 30004;
scanf("%d%d", &n, &m);
for (int i = 0; i < m; i++) {
	scanf("%d%d", &x, &y);
	if (n % x == 0) {
		a = n / x * y;
	} else {
		a = n / x * y + y;
	}
	if (a < M) {
		M = a;
	}
}
printf("%d", M);
return 0;

}//有点像买铅笔的变形题。