N=int(input())
C=[[int(i) for i  in input(). split ()] for i in range(N)]
S=0
for i in range(N):
    if C[i][0]/2!=C[i][0]//2:
        e=C[i][0]//2+1
    else:
        e=C[i][0]/2
    e=e*C[i][1]
    S+=e
print (int(S))