[Python] softeer.ai 금고털이

import sys
input = sys.stdin.readline

W, N = map(int, input().split())
stone = []

for _ in range(N):
    mp = list(map(int, input().split()))
    stone.append(mp)

stone.sort(key = lambda x : -x[1])

price = 0

for m, p in stone:
    if m <= W:
        W -= m
        price += m * p
    else:
        price += W * p
        break

print(price)