Botswana, a country of some 648,000 people unevenly distributed over more than 600,000 square kilometres, is an extremely poor nation whose estimated income per capita is less than $100 per annum. It is land-locked and dependent upon transport routes through neighbouring South Africa and Rhodesia. Frequent droughts cause major losses in Botswana's cattle herds, whose meat provides the country with its major export earnings. According to the 1964 census, almost 228,000 of the 250,000 workers were in agriculture, forestry, hunting, and fishing, and a 1967–8 survey found only 28,148 in salaried employment. During 1967 an additional 22,735 Batswana were working in the mines of South Africa. Of the present number of jobs (estimated at 2,000) requiring the minimum educational standard of School Certificate, less than one-quarter are currently filled by Botswana citizens. In 1967 some 258 students – 55 per cent of those who sat for the examination – received the Junior Certificate (after three years of secondary education) and 66 students - 80 per cent of those who sat – received the Cambridge School Certificate (after five years of secondary education).