import sys
from math import sqrt, pi, sin, factorial
inp = lambda : sys.stdin.readline()[:-1].strip()
mii = lambda x = " " : [*map(int,inp().split(x))]
mfi = lambda x = " " : [*map(float,inp().split(x))]
ii = lambda : int(inp())
fi = lambda : float(inp())
p = print
def solve():
m, h = mii() # 우유 단위당 행복, 꿀 단위당 행복
n = ii()
l = [mii() for _ in range(n)]
ans = 0
for a, b in l:
ans += max(a * m, b * h)
p(ans)
if __name__ == "__main__":
tc = 1
for t in range(1, tc+1):
solve()