879A - Borya's Diagnosis - CodeForces Solution

x = lambda: map(int,input().split())
n, = x()
cnt = 0; c = 0
for i in range(n):
    a,b = x()
    j = c+1; cnt2 = 0
    while True:
        if(j in range(a,10**20,b)): cnt += (j-c); c = j;break
        if(j%b and cnt2 != 0): cnt2 = 1
        if(cnt2): j += b
        else: j += 1
print(cnt)