n = int(input())
ans = None
valid = True

for _ in range(n):
    x, y = map(int, input().split())
    diff = y - x
    # Check if diff is non-positive
    if diff <= 0:
        valid = False
    # Check if all diffs are the same
    if ans is None:
        ans = diff
    else:
        if diff != ans:
            valid = False

if valid and ans > 0:
    print(ans)
else:
    print(-1)