Paris is the capital and most populous city of France. Excluding the outlying parks of Bois de Boulogne and Bois de Vincennes, Paris occupies an oval measuring about 87 km2 (34 sq mi) in area, enclosed by the 35 km (22 mi) ring road, the Boulevard Périphérique. Paris is located in the north-bending arc of the river Seine, spread widely on both banks of the river, and includes two inhabited islands, the Île Saint-Louis and the larger Île de la Cité, which forms the oldest part of the city. The river’s mouth on the English Channel (La Manche) is about 233 mi (375 km) downstream of the city. Overall, the city is relatively flat, and the lowest point is 35 m (115 ft) above sea level. Paris has several prominent hills, of which the highest is Montmartre at 130 m (427 ft). Montmartre gained its name from the martyrdom of Saint Denis, first bishop of Paris atop the "Mons Martyrum" (Martyr's mound) in 250.