Well... integral from t0 to t1 of exp(at+b) dt = (exp(at1+b)-exp(a*t2+b))/a i.e. the difference between the endpoints times the time needed to increase by a factor of e... a 6-million-fold increase is about 22.5 doublings (knowing 2^20 = 1 million), hence about 15 factors of e (knowing that ln 2 = 0.7) i.e. about one in 150... hence the total number of tourists is about 1 billion (about six times less than Rhwawn's estimate -- my eyeballs had told me it would be about one third... close enough!)