N = int(input())
I = [list(map(int, input().split())) for _ in range(N)]
mod = 998244353
l0, r0 = I[0]
ans = 0
for i in range(1, N):
    li, ri = I[i]
    L, R = min(max(li, l0), r0+1), min(ri, r0)
    ans += max(0, L-l0)
    if L <= R:
        ans += max(0, (2*ri-L-R)*(R-L+1)//2)*pow(ri-li+1, mod-2, mod)
    ans %= mod
ans = (1+ans*pow(r0-l0+1, mod-2, mod))%mod
print(ans)