if __name__ == '__main__':
while True:
nravg = sys.stdin.readline().strip()
if not nravg:
break
n,r,avg = [int(i) for i in nravg.split(' ')]
nums = []
alls = n*avg
used =0
for i in range(n):
tp = [int(i) for i in sys.stdin.readline().strip().split(' ')]
nums.append(tp)
# print(nums)
used += tp[0]
# print(used)
alls -= used
need = 0
nums.sort(key = lambda p:p[1])
# print(nums)
if alls>0:
for i in range(n):
curNeed = min(alls,r-nums[i][0])
alls-=curNeed
need+= curNeed* nums[i][1]
if alls == 0:
break
print(need)
else:
print(0)